diff --git a/src/neuroconv/tools/data_transfers/_dandi.py b/src/neuroconv/tools/data_transfers/_dandi.py
index 11d38b0cb..0bc0b8fb8 100644
--- a/src/neuroconv/tools/data_transfers/_dandi.py
+++ b/src/neuroconv/tools/data_transfers/_dandi.py
@@ -90,6 +90,10 @@ def automatic_dandi_upload(
             with NWBHDF5IO(path=organized_nwbfile, mode="r") as io:
                 nwbfile = io.read()
                 session_id = nwbfile.session_id
+
+            # Replace invalid characters with underscores
+            session_id = session_id.replace("/", "_").replace("\\", "_").replace(":", "_")
+
             dandi_stem = organized_nwbfile.stem
             dandi_stem_split = dandi_stem.split("_")
             dandi_stem_split.insert(1, f"ses-{session_id}")
diff --git a/tests/test_minimal/test_tools/dandi_transfer_tools.py b/tests/test_minimal/test_tools/dandi_transfer_tools.py
index da35725a0..e38c75501 100644
--- a/tests/test_minimal/test_tools/dandi_transfer_tools.py
+++ b/tests/test_minimal/test_tools/dandi_transfer_tools.py
@@ -80,3 +80,47 @@ def test_automatic_dandi_upload_non_parallel_non_threaded(tmp_path):
         number_of_jobs=1,
         number_of_threads=1,
     )
+
+
+@pytest.mark.skipif(
+    not HAVE_DANDI_KEY,
+    reason="You must set your DANDI_API_KEY to run this test!",
+)
+def test_automatic_dandi_upload_invalid_session_id_chars(tmp_path):
+    """Test that invalid characters in session_id are replaced with underscores during DANDI upload."""
+    nwb_folder_path = tmp_path / "test_nwb"
+    nwb_folder_path.mkdir()
+
+    # Create metadata with session_id containing invalid characters
+    metadata = get_default_nwbfile_metadata()
+    invalid_chars_session_id = f"test/invalid\\chars:session-id-{datetime.now().strftime('%Y%m%d%H%M%S')}"
+    expected_session_id = invalid_chars_session_id.replace("/", "_").replace("\\", "_").replace(":", "_")
+
+    metadata["NWBFile"].update(
+        session_start_time=datetime.now().astimezone(),
+        session_id=invalid_chars_session_id,
+    )
+    metadata.update(Subject=dict(subject_id="foo", species="Mus musculus", age="P1D", sex="U"))
+
+    # Create NWB file
+    nwb_file_path = nwb_folder_path / "test_invalid_chars.nwb"
+    with NWBHDF5IO(path=nwb_file_path, mode="w") as io:
+        io.write(make_nwbfile_from_metadata(metadata=metadata))
+
+    # Upload to DANDI
+    organized_nwbfiles = automatic_dandi_upload(
+        dandiset_id="200560",
+        nwb_folder_path=nwb_folder_path,
+        staging=True,
+        cleanup=False,  # Don't clean up so we can check the renamed file
+    )
+
+    # Verify that at least one file was organized
+    assert len(organized_nwbfiles) > 0
+
+    # Check that the organized file name contains the sanitized session_id
+    for file_path in organized_nwbfiles:
+        file_path = Path(file_path)
+        assert (
+            "ses-" + expected_session_id in file_path.stem
+        ), f"Expected sanitized session ID not found in {file_path.stem}"