diff --git a/docs/changelog/3396.bugfix.rst b/docs/changelog/3396.bugfix.rst
new file mode 100644
index 0000000000..cb13f44bd0
--- /dev/null
+++ b/docs/changelog/3396.bugfix.rst
@@ -0,0 +1 @@
+Fix a bug, that prevented the installation of extra dependencies, which contain an underscore in their identifier.
diff --git a/src/tox/tox_env/python/virtual_env/package/pyproject.py b/src/tox/tox_env/python/virtual_env/package/pyproject.py
index b70fc677ca..f9d9cc4ff0 100644
--- a/src/tox/tox_env/python/virtual_env/package/pyproject.py
+++ b/src/tox/tox_env/python/virtual_env/package/pyproject.py
@@ -13,6 +13,7 @@
 
 from cachetools import cached
 from packaging.requirements import Requirement
+from packaging.utils import canonicalize_name
 from pyproject_api import (
     BackendFailed,
     CmdStatus,
@@ -313,7 +314,7 @@ def _load_deps_from_static(self, for_env: EnvConfigSet) -> list[Requirement] | N
         ]
         optional_deps = project.get("optional-dependencies", {})
         for extra, reqs in optional_deps.items():
-            deps_with_markers.extend((Requirement(req), {extra}) for req in (reqs or []))
+            deps_with_markers.extend((Requirement(req), {canonicalize_name(extra)}) for req in (reqs or []))
         return dependencies_with_extras_from_markers(
             deps_with_markers=deps_with_markers,
             extras=extras,