diff --git a/.github/workflows/build_and_test.yml b/.github/workflows/build_and_test.yml
index 7cec8a9..9f70ea2 100644
--- a/.github/workflows/build_and_test.yml
+++ b/.github/workflows/build_and_test.yml
@@ -17,10 +17,8 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-22.04, macos-latest]
-        python-version: ["3.9", "3.10", "3.11"]
+        python-version: ["3.10", "3.11", "3.12"]
         exclude:
-          - os: macos-latest
-            python-version: '3.9'
           - os: macos-latest
             python-version: '3.10'
     name: "Core, Python ${{ matrix.python-version }}, ${{ matrix.os }}"
diff --git a/pyproject.toml b/pyproject.toml
index 91a8cfe..c9d308a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,9 +25,9 @@ authors = [
 ]
 
 # Required Python version and dependencies
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 dependencies = [
-    "fractal-tasks-core==1.3.4"
+    "fractal-tasks-core==1.4.0","ngio==0.1.4",
 ]
 
 # Optional dependencies (e.g. for `pip install -e ".[dev]"`, see
diff --git a/src/fractal_helper_tasks/__FRACTAL_MANIFEST__.json b/src/fractal_helper_tasks/__FRACTAL_MANIFEST__.json
index 99941bc..60cd415 100644
--- a/src/fractal_helper_tasks/__FRACTAL_MANIFEST__.json
+++ b/src/fractal_helper_tasks/__FRACTAL_MANIFEST__.json
@@ -33,7 +33,7 @@
             "default": false,
             "title": "Overwrite Input",
             "type": "boolean",
-            "description": "Whether"
+            "description": "Whether the existing iamge should be overwritten with the new OME-Zarr without the T dimension."
           }
         },
         "required": [
@@ -128,6 +128,73 @@
         "title": "Convert2dSegmentationTo3d"
       },
       "docs_link": "https://github.com/jluethi/fractal-helper-tasks"
+    },
+    {
+      "name": "Rechunk OME-Zarr",
+      "tags": [
+        "Rechunking",
+        "Many files"
+      ],
+      "docs_info": "### Purpose\n- Rechunks OME-Zarr to new chunking parameters: Changes whether the array is stored as many small files or few larger files.\n- Optionally applies the same rechunking to label images.\n\n### Outputs\n- A **new Zarr image** that is rechunked.\n",
+      "executable_parallel": "rechunk_zarr.py",
+      "meta_parallel": {
+        "cpus_per_task": 1,
+        "mem": 4000
+      },
+      "args_schema_parallel": {
+        "additionalProperties": false,
+        "properties": {
+          "zarr_url": {
+            "title": "Zarr Url",
+            "type": "string",
+            "description": "Path or url to the individual OME-Zarr image to be processed. (standard argument for Fractal tasks, managed by Fractal server)."
+          },
+          "chunk_sizes": {
+            "additionalProperties": {
+              "type": "integer"
+            },
+            "title": "Chunk Sizes",
+            "type": "object",
+            "description": "Dictionary of chunk sizes to adapt. One can set any of the t, c, z, y, x axes that exist in the input image to be resized to a different chunk size. For example, {\"y\": 4000, \"x\": 4000} will set a new x & y chunking while maintaining the other chunk sizes. {\"z\": 10} will just change the Z chunking while keeping all other chunk sizes the same as the input."
+          },
+          "suffix": {
+            "default": "rechunked",
+            "title": "Suffix",
+            "type": "string",
+            "description": "Suffix of the rechunked image."
+          },
+          "rechunk_labels": {
+            "default": true,
+            "title": "Rechunk Labels",
+            "type": "boolean",
+            "description": "Whether to apply the same rechunking to all label images of the OME-Zarr as well."
+          },
+          "rebuild_pyramids": {
+            "default": true,
+            "title": "Rebuild Pyramids",
+            "type": "boolean",
+            "description": "Whether pyramids are built fresh in the rechunked image. This has a small performance overhead, but ensures that this task is save against off-by-one issues when pyramid levels aren't easily downsampled by 2."
+          },
+          "overwrite_input": {
+            "default": true,
+            "title": "Overwrite Input",
+            "type": "boolean",
+            "description": "Whether the old image without rechunking should be overwritten (to avoid duplicating the data needed)."
+          },
+          "overwrite": {
+            "default": false,
+            "title": "Overwrite",
+            "type": "boolean",
+            "description": "Whether to overwrite potential pre-existing output with the name zarr_url_suffix."
+          }
+        },
+        "required": [
+          "zarr_url"
+        ],
+        "type": "object",
+        "title": "RechunkZarr"
+      },
+      "docs_link": "https://github.com/jluethi/fractal-helper-tasks"
     }
   ],
   "has_args_schemas": true,
diff --git a/src/fractal_helper_tasks/dev/docs_info/rechunk_zarr.md b/src/fractal_helper_tasks/dev/docs_info/rechunk_zarr.md
new file mode 100644
index 0000000..6f79c9d
--- /dev/null
+++ b/src/fractal_helper_tasks/dev/docs_info/rechunk_zarr.md
@@ -0,0 +1,6 @@
+### Purpose
+- Rechunks OME-Zarr to new chunking parameters: Changes whether the array is stored as many small files or few larger files.
+- Optionally applies the same rechunking to label images.
+
+### Outputs
+- A **new Zarr image** that is rechunked.
diff --git a/src/fractal_helper_tasks/dev/task_list.py b/src/fractal_helper_tasks/dev/task_list.py
index 4aa71f5..8289643 100644
--- a/src/fractal_helper_tasks/dev/task_list.py
+++ b/src/fractal_helper_tasks/dev/task_list.py
@@ -22,4 +22,14 @@
         ],
         docs_info="file:docs_info/2d_to_3d.md",
     ),
+    ParallelTask(
+        name="Rechunk OME-Zarr",
+        executable="rechunk_zarr.py",
+        meta={"cpus_per_task": 1, "mem": 4000},
+        tags=[
+            "Rechunking",
+            "Many files",
+        ],
+        docs_info="file:docs_info/rechunk_zarr.md",
+    ),
 ]
diff --git a/src/fractal_helper_tasks/drop_t_dimension.py b/src/fractal_helper_tasks/drop_t_dimension.py
index 9c59de5..782c34b 100644
--- a/src/fractal_helper_tasks/drop_t_dimension.py
+++ b/src/fractal_helper_tasks/drop_t_dimension.py
@@ -53,7 +53,8 @@ def drop_t_dimension(
             (standard argument for Fractal tasks, managed by Fractal server).
         suffix: Suffix to be used for the new Zarr image. If overwrite_input
             is True, this file is only temporary.
-        overwrite_input: Whether
+        overwrite_input: Whether the existing iamge should be overwritten with
+            the new OME-Zarr without the T dimension.
     """
     # Normalize zarr_url
     zarr_url_old = zarr_url.rstrip("/")
diff --git a/src/fractal_helper_tasks/rechunk_zarr.py b/src/fractal_helper_tasks/rechunk_zarr.py
new file mode 100644
index 0000000..5c48f1f
--- /dev/null
+++ b/src/fractal_helper_tasks/rechunk_zarr.py
@@ -0,0 +1,145 @@
+# Copyright 2025 (C) BioVisionCenter, University of Zurich
+#
+# Original authors:
+# Joel Lüthi <joel.luethi@uzh.ch>
+"""Rechunk an existing Zarr."""
+
+import logging
+import os
+import shutil
+from typing import Any, Optional
+
+import ngio
+from pydantic import validate_call
+
+from fractal_helper_tasks.utils import normalize_chunk_size_dict, rechunk_label
+
+logger = logging.getLogger(__name__)
+
+
+@validate_call
+def rechunk_zarr(
+    *,
+    zarr_url: str,
+    chunk_sizes: Optional[dict[str, Optional[int]]] = None,
+    suffix: str = "rechunked",
+    rechunk_labels: bool = True,
+    rebuild_pyramids: bool = True,
+    overwrite_input: bool = True,
+    overwrite: bool = False,
+) -> dict[str, Any]:
+    """Drops singleton t dimension.
+
+    Args:
+        zarr_url: Path or url to the individual OME-Zarr image to be processed.
+            (standard argument for Fractal tasks, managed by Fractal server).
+        chunk_sizes: Dictionary of chunk sizes to adapt. One can set any of
+            the t, c, z, y, x axes that exist in the input image to be resized
+            to a different chunk size. For example, {"y": 4000, "x": 4000}
+            will set a new x & y chunking while maintaining the other chunk
+            sizes. {"z": 10} will just change the Z chunking while keeping
+            all other chunk sizes the same as the input.
+        suffix: Suffix of the rechunked image.
+        rechunk_labels: Whether to apply the same rechunking to all label
+            images of the OME-Zarr as well.
+        rebuild_pyramids: Whether pyramids are built fresh in the rechunked
+            image. This has a small performance overhead, but ensures that
+            this task is save against off-by-one issues when pyramid levels
+            aren't easily downsampled by 2.
+        overwrite_input: Whether the old image without rechunking should be
+            overwritten (to avoid duplicating the data needed).
+        overwrite: Whether to overwrite potential pre-existing output with the
+            name zarr_url_suffix.
+    """
+    logger.info(f"Running `rechunk_zarr` on {zarr_url=} with {chunk_sizes=}.")
+
+    chunk_sizes = normalize_chunk_size_dict(chunk_sizes)
+
+    rechunked_zarr_url = zarr_url + f"_{suffix}"
+    ngff_image = ngio.NgffImage(zarr_url)
+    pyramid_paths = ngff_image.levels_paths
+    highest_res_img = ngff_image.get_image()
+    axes_names = highest_res_img.dataset.on_disk_axes_names
+    chunks = highest_res_img.on_disk_dask_array.chunks
+
+    # Compute the chunksize tuple
+    new_chunksize = [c[0] for c in chunks]
+    logger.info(f"Initial chunk sizes were: {chunks}")
+    # Overwrite chunk_size with user-set chunksize
+    for i, axis in enumerate(axes_names):
+        if axis in chunk_sizes:
+            if chunk_sizes[axis] is not None:
+                new_chunksize[i] = chunk_sizes[axis]
+
+    for axis in chunk_sizes:
+        if axis not in axes_names:
+            raise NotImplementedError(
+                f"Rechunking with {axis=} is specified, but the OME-Zarr only "
+                f"has the following axes: {axes_names}"
+            )
+
+    logger.info(f"Chunk sizes after rechunking will be: {new_chunksize=}")
+
+    new_ngff_image = ngff_image.derive_new_image(
+        store=rechunked_zarr_url,
+        name=ngff_image.image_meta.name,
+        overwrite=overwrite,
+        copy_labels=not rechunk_labels,
+        copy_tables=True,
+        chunks=new_chunksize,
+    )
+
+    ngff_image = ngio.NgffImage(zarr_url)
+
+    if rebuild_pyramids:
+        # Set the highest resolution, then consolidate to build a new pyramid
+        new_ngff_image.get_image(highest_resolution=True).set_array(
+            ngff_image.get_image(highest_resolution=True).on_disk_dask_array
+        )
+        new_ngff_image.get_image(highest_resolution=True).consolidate()
+    else:
+        for path in pyramid_paths:
+            new_ngff_image.get_image(path=path).set_array(
+                ngff_image.get_image(path=path).on_disk_dask_array
+            )
+
+    # Copy labels
+    if rechunk_labels:
+        chunk_sizes["c"] = None
+        label_names = ngff_image.labels.list()
+        for label in label_names:
+            rechunk_label(
+                orig_ngff_image=ngff_image,
+                new_ngff_image=new_ngff_image,
+                label=label,
+                chunk_sizes=chunk_sizes,
+                overwrite=overwrite,
+                rebuild_pyramids=rebuild_pyramids,
+            )
+
+    if overwrite_input:
+        os.rename(zarr_url, f"{zarr_url}_tmp")
+        os.rename(rechunked_zarr_url, zarr_url)
+        shutil.rmtree(f"{zarr_url}_tmp")
+        return
+    else:
+        output = dict(
+            image_list_updates=[
+                dict(
+                    zarr_url=rechunked_zarr_url,
+                    origin=zarr_url,
+                    types=dict(rechunked=True),
+                )
+            ],
+            filters=dict(types=dict(rechunked=True)),
+        )
+        return output
+
+
+if __name__ == "__main__":
+    from fractal_tasks_core.tasks._utils import run_fractal_task
+
+    run_fractal_task(
+        task_function=rechunk_zarr,
+        logger_name=logger.name,
+    )
diff --git a/src/fractal_helper_tasks/utils.py b/src/fractal_helper_tasks/utils.py
new file mode 100644
index 0000000..3c5c609
--- /dev/null
+++ b/src/fractal_helper_tasks/utils.py
@@ -0,0 +1,114 @@
+# Copyright 2025 (C) BioVisionCenter, University of Zurich
+#
+# Original authors:
+# Joel Lüthi <joel.luethi@uzh.ch>
+"""Utils for helper tasks."""
+
+from typing import Optional
+
+import ngio
+from ngio.core.utils import create_empty_ome_zarr_label
+
+
+def normalize_chunk_size_dict(chunk_sizes: dict[str, Optional[int]]):
+    """Converts all chunk_size axes names to lower case and assert validity.
+
+    Args:
+        chunk_sizes: Dictionary of chunk sizes that should be adapted. Can
+            contain new chunk sizes for t, c, z, y & x.
+
+    Returns:
+        chunk_sizes_norm: Normalized chunk_sizes dict.
+    """
+    chunk_sizes = chunk_sizes or {}
+    chunk_sizes_norm = {}
+    for key, value in chunk_sizes.items():
+        chunk_sizes_norm[key.lower()] = value
+
+    valid_axes = ["t", "c", "z", "y", "x"]
+    for axis in chunk_sizes_norm:
+        if axis not in valid_axes:
+            raise ValueError(
+                f"Axis {axis} is not supported. Valid axes choices are "
+                f"{valid_axes}."
+            )
+    return chunk_sizes_norm
+
+
+def rechunk_label(
+    orig_ngff_image: ngio.NgffImage,
+    new_ngff_image: ngio.NgffImage,
+    label: str,
+    chunk_sizes: list[int],
+    overwrite: bool = False,
+    rebuild_pyramids: bool = True,
+):
+    """Saves a rechunked label image into a new OME-Zarr
+
+    The label image is based on an existing label image in another OME-Zarr.
+
+    Args:
+        orig_ngff_image: Original OME-Zarr that contains the label image
+        new_ngff_image: OME-Zarr to which the rechunked label image should be
+            added.
+        label: Name of the label image.
+        chunk_sizes: New chunk sizes that should be applied
+        overwrite: Whether the label image in `new_ngff_image` should be
+            overwritten if it already exists.
+        rebuild_pyramids: Whether pyramids are built fresh in the rechunked
+            label image. This has a small performance overhead, but ensures
+            that this task is save against off-by-one issues when pyramid
+            levels aren't easily downsampled by 2.
+    """
+    old_label = orig_ngff_image.labels.get_label(name=label)
+    label_level_paths = orig_ngff_image.labels.levels_paths(name=label)
+    # Compute the chunksize tuple
+    chunks = old_label.on_disk_dask_array.chunks
+    new_chunksize = [c[0] for c in chunks]
+    # Overwrite chunk_size with user-set chunksize
+    for i, axis in enumerate(old_label.dataset.on_disk_axes_names):
+        if axis in chunk_sizes:
+            if chunk_sizes[axis] is not None:
+                new_chunksize[i] = chunk_sizes[axis]
+    create_empty_ome_zarr_label(
+        store=new_ngff_image.store + "/" + "labels" + "/" + label,
+        on_disk_shape=old_label.on_disk_shape,
+        chunks=new_chunksize,
+        dtype=old_label.on_disk_dask_array.dtype,
+        on_disk_axis=old_label.dataset.on_disk_axes_names,
+        pixel_sizes=old_label.dataset.pixel_size,
+        xy_scaling_factor=old_label.metadata.xy_scaling_factor,
+        z_scaling_factor=old_label.metadata.z_scaling_factor,
+        time_spacing=old_label.dataset.time_spacing,
+        time_units=old_label.dataset.time_axis_unit,
+        levels=label_level_paths,
+        name=label,
+        overwrite=overwrite,
+        version=old_label.metadata.version,
+    )
+
+    # Fill in labels .attrs to contain the label name
+    list_of_labels = new_ngff_image.labels.list()
+    if label not in list_of_labels:
+        new_ngff_image.labels._label_group.attrs["labels"] = [
+            *list_of_labels,
+            label,
+        ]
+
+    if rebuild_pyramids:
+        # Set the highest resolution, then consolidate to build a new pyramid
+        new_ngff_image.labels.get_label(name=label, highest_resolution=True).set_array(
+            orig_ngff_image.labels.get_label(
+                name=label, highest_resolution=True
+            ).on_disk_dask_array
+        )
+        new_ngff_image.labels.get_label(
+            name=label, highest_resolution=True
+        ).consolidate()
+    else:
+        for label_path in label_level_paths:
+            new_ngff_image.labels.get_label(name=label, path=label_path).set_array(
+                orig_ngff_image.labels.get_label(
+                    name=label, path=label_path
+                ).on_disk_dask_array
+            )
diff --git a/tests/test_rechunk_zarr.py b/tests/test_rechunk_zarr.py
new file mode 100644
index 0000000..2e7deb3
--- /dev/null
+++ b/tests/test_rechunk_zarr.py
@@ -0,0 +1,134 @@
+"""Test copy 2D to 3D segmentation."""
+
+import ngio
+import pytest
+
+from fractal_helper_tasks.rechunk_zarr import (
+    rechunk_zarr,
+)
+
+
+@pytest.mark.parametrize(
+    "chunk_sizes, output_chunk_sizes",
+    [
+        ({"x": 1000, "y": 1000}, [1, 1, 1000, 1000]),
+        ({"X": 1000, "Y": 1000}, [1, 1, 1000, 1000]),
+        ({"x": 6000, "y": 6000}, [1, 1, 2160, 5120]),
+        ({}, [1, 1, 2160, 2560]),
+        ({"x": None, "y": None}, [1, 1, 2160, 2560]),
+        ({"z": 10}, [1, 1, 2160, 2560]),
+        ({"Z": 10}, [1, 1, 2160, 2560]),
+    ],
+)
+def test_rechunk_2d(tmp_zenodo_zarr: list[str], chunk_sizes, output_chunk_sizes):
+    zarr_url = f"{tmp_zenodo_zarr[1]}/B/03/0"
+
+    rechunk_zarr(
+        zarr_url=zarr_url,
+        chunk_sizes=chunk_sizes,
+    )
+
+    chunks = ngio.NgffImage(zarr_url).get_image().on_disk_dask_array.chunks
+    chunk_sizes = [c[0] for c in chunks]
+    assert chunk_sizes == output_chunk_sizes
+
+
+@pytest.mark.parametrize(
+    "chunk_sizes, output_chunk_sizes",
+    [
+        ({"x": None, "y": None}, [1, 1, 2160, 2560]),
+        ({"z": 10}, [1, 2, 2160, 2560]),
+    ],
+)
+def test_rechunk_3d(tmp_zenodo_zarr: list[str], chunk_sizes, output_chunk_sizes):
+    zarr_url = f"{tmp_zenodo_zarr[0]}/B/03/0"
+
+    rechunk_zarr(
+        zarr_url=zarr_url,
+        chunk_sizes=chunk_sizes,
+    )
+
+    chunks = ngio.NgffImage(zarr_url).get_image().on_disk_dask_array.chunks
+    chunk_sizes = [c[0] for c in chunks]
+    assert chunk_sizes == output_chunk_sizes
+
+
+@pytest.mark.parametrize(
+    "rechunk_labels, output_chunk_sizes",
+    [
+        (True, [1, 300, 300]),
+        (False, [1, 540, 1280]),
+    ],
+)
+def test_rechunk_labels(tmp_zenodo_zarr: list[str], rechunk_labels, output_chunk_sizes):
+    zarr_url = f"{tmp_zenodo_zarr[1]}/B/03/0"
+    chunk_sizes = {"x": 300, "y": 300}
+
+    rechunk_zarr(
+        zarr_url=zarr_url,
+        chunk_sizes=chunk_sizes,
+        rechunk_labels=rechunk_labels,
+    )
+    chunks = (
+        ngio.NgffImage(zarr_url)
+        .labels.get_label(name="nuclei", path="0")
+        .on_disk_dask_array.chunks
+    )
+    chunk_sizes = [c[0] for c in chunks]
+    assert chunk_sizes == output_chunk_sizes
+
+
+@pytest.mark.parametrize(
+    "chunk_sizes, error_axes",
+    [
+        ({"test": 1000, "y": 1000}, "test"),
+        ({"u": 1000}, "u"),
+    ],
+)
+def test_invalid_axis(tmp_zenodo_zarr: list[str], chunk_sizes, error_axes):
+    zarr_url = f"{tmp_zenodo_zarr[1]}/B/03/0"
+
+    with pytest.raises(ValueError) as e:
+        rechunk_zarr(
+            zarr_url=zarr_url,
+            chunk_sizes=chunk_sizes,
+        )
+    assert f"Axis {error_axes} is not supported" in str(e.value)
+
+
+def test_rechunk_no_overwrite_input(tmp_zenodo_zarr: list[str]):
+    zarr_url = f"{tmp_zenodo_zarr[1]}/B/03/0"
+    suffix = "rechunked_custom"
+    new_zarr_url = f"{zarr_url}_{suffix}"
+    overwrite_input = False
+    chunk_sizes = {"x": 1000, "y": 1000}
+    output_chunk_sizes = [1, 1, 1000, 1000]
+    original_chunk_sizes = [1, 1, 2160, 2560]
+
+    output = rechunk_zarr(
+        zarr_url=zarr_url,
+        chunk_sizes=chunk_sizes,
+        suffix=suffix,
+        overwrite_input=overwrite_input,
+    )
+    expected_output = dict(
+        image_list_updates=[
+            dict(
+                zarr_url=new_zarr_url,
+                origin=zarr_url,
+                types=dict(rechunked=True),
+            )
+        ],
+        filters=dict(types=dict(rechunked=True)),
+    )
+    assert expected_output == output
+
+    # Existing zarr should be unchanged, but new zarr should have
+    # expected chunking
+    chunks = ngio.NgffImage(zarr_url).get_image().on_disk_dask_array.chunks
+    chunk_sizes = [c[0] for c in chunks]
+    assert chunk_sizes == original_chunk_sizes
+
+    chunks = ngio.NgffImage(new_zarr_url).get_image().on_disk_dask_array.chunks
+    chunk_sizes = [c[0] for c in chunks]
+    assert chunk_sizes == output_chunk_sizes