diff --git a/docs/changelog/137275.yaml b/docs/changelog/137275.yaml
new file mode 100644
index 0000000000000..d39b968d358e2
--- /dev/null
+++ b/docs/changelog/137275.yaml
@@ -0,0 +1,6 @@
+pr: 137275
+summary: "Reindex-from-remote: Fail on manual slicing param"
+area: Indices APIs
+type: bug
+issues:
+ - 136269
diff --git a/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java b/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
index 24fc46bb88792..edce26e55dfd1 100644
--- a/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
+++ b/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
@@ -115,6 +115,12 @@ public ActionRequestValidationException validate() {
             if (getSlices() == AbstractBulkByScrollRequest.AUTO_SLICES || getSlices() > 1) {
                 e = addValidationError("reindex from remote sources doesn't support slices > 1 but was [" + getSlices() + "]", e);
             }
+            if (getSearchRequest().source().slice() != null) {
+                e = addValidationError(
+                    "reindex from remote sources doesn't support source.slice but was [" + getSearchRequest().source().slice() + "]",
+                    e
+                );
+            }
             if (getRemoteInfo().getUsername() != null && getRemoteInfo().getPassword() == null) {
                 e = addValidationError("reindex from remote source included username but not password", e);
             }
diff --git a/server/src/test/java/org/elasticsearch/index/reindex/ReindexRequestTests.java b/server/src/test/java/org/elasticsearch/index/reindex/ReindexRequestTests.java
index 2204a0bb3b2b0..44f867124be39 100644
--- a/server/src/test/java/org/elasticsearch/index/reindex/ReindexRequestTests.java
+++ b/server/src/test/java/org/elasticsearch/index/reindex/ReindexRequestTests.java
@@ -162,7 +162,7 @@ public void testReindexFromRemoteDoesNotSupportSearchQuery() {
         );
     }
 
-    public void testReindexFromRemoteDoesNotSupportSlices() {
+    public void testReindexFromRemoteDoesNotSupportSlicesParameterGreaterThan1() {
         ReindexRequest reindex = newRequest();
         reindex.setRemoteInfo(
             new RemoteInfo(
@@ -178,6 +178,7 @@ public void testReindexFromRemoteDoesNotSupportSlices() {
                 RemoteInfo.DEFAULT_CONNECT_TIMEOUT
             )
         );
+        // Enable automatic slicing with a random number of slices greater than 1 (like setting the slices URL parameter):
         reindex.setSlices(between(2, Integer.MAX_VALUE));
         ActionRequestValidationException e = reindex.validate();
         assertEquals(
@@ -186,6 +187,60 @@ public void testReindexFromRemoteDoesNotSupportSlices() {
         );
     }
 
+    public void testReindexFromRemoteDoesNotSupportSlicesParameterSetToAuto() {
+        ReindexRequest reindex = newRequest();
+        reindex.setRemoteInfo(
+            new RemoteInfo(
+                randomAlphaOfLength(5),
+                randomAlphaOfLength(5),
+                between(1, Integer.MAX_VALUE),
+                null,
+                matchAll,
+                null,
+                null,
+                emptyMap(),
+                RemoteInfo.DEFAULT_SOCKET_TIMEOUT,
+                RemoteInfo.DEFAULT_CONNECT_TIMEOUT
+            )
+        );
+        // Enable automatic slicing with an automatically chosen number of slices (like setting the slices URL parameter to "auto"):
+        reindex.setSlices(AbstractBulkByScrollRequest.AUTO_SLICES);
+        ActionRequestValidationException e = reindex.validate();
+        assertEquals(
+            "Validation Failed: 1: reindex from remote sources doesn't support slices > 1 but was [" + reindex.getSlices() + "];",
+            e.getMessage()
+        );
+    }
+
+    public void testReindexFromRemoteDoesNotSupportSlicesSourceField() {
+        ReindexRequest reindex = newRequest();
+        reindex.setRemoteInfo(
+            new RemoteInfo(
+                randomAlphaOfLength(5),
+                randomAlphaOfLength(5),
+                between(1, Integer.MAX_VALUE),
+                null,
+                matchAll,
+                null,
+                null,
+                emptyMap(),
+                RemoteInfo.DEFAULT_SOCKET_TIMEOUT,
+                RemoteInfo.DEFAULT_CONNECT_TIMEOUT
+            )
+        );
+        // Enable manual slicing (like setting source.slice.max and source.slice.id in the request body):
+        int numSlices = randomIntBetween(2, Integer.MAX_VALUE);
+        int sliceId = randomIntBetween(0, numSlices - 1);
+        reindex.getSearchRequest().source().slice(new SliceBuilder(sliceId, numSlices));
+        ActionRequestValidationException e = reindex.validate();
+        assertEquals(
+            "Validation Failed: 1: reindex from remote sources doesn't support source.slice but was ["
+                + reindex.getSearchRequest().source().slice()
+                + "];",
+            e.getMessage()
+        );
+    }
+
     public void testReindexFromRemoteRejectsUsernameWithNoPassword() {
         ReindexRequest reindex = newRequest();
         reindex.setRemoteInfo(