diff --git a/CHANGELOG.md b/CHANGELOG.md
index 58fb40a..2a99b30 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added `--overwrite` flag to overwrite existing ZIM file if it exists (#399)
+
 ### Changed
 - Fix issues preventing interrupted crawls from being resumed. (#499)
   - Ensure build directory is used explicitly instead of a randomized subdirectory when passed, and pre-create it if it does not exist.
diff --git a/offliner-definition.json b/offliner-definition.json
index 4000466..4bb68b5 100644
--- a/offliner-definition.json
+++ b/offliner-definition.json
@@ -970,6 +970,12 @@
       "alias": "name",
       "pattern": "^([a-z0-9\\-\\.]+_)([a-z\\-]+_)([a-z0-9\\-\\.]+)$",
       "relaxedPattern": "^[A-Za-z0-9._-]+$"
+    },
+    "overwrite": {
+      "type": "boolean",
+      "required": false,
+      "title": "Overwrite",
+      "description": "Whether to overwrite existing ZIM file if it exists"
     }
   }
 }
diff --git a/src/zimit/zimit.py b/src/zimit/zimit.py
index e982cbd..b205007 100755
--- a/src/zimit/zimit.py
+++ b/src/zimit/zimit.py
@@ -849,6 +849,9 @@ def run(raw_args):
         warc2zim_args.append("--lang")
         warc2zim_args.append(known_args.zim_lang)
 
+    if known_args.overwrite:
+        warc2zim_args.append("--overwrite")
+
     logger.info("----------")
     logger.info("Testing warc2zim args")
     logger.info("Running: warc2zim " + " ".join(warc2zim_args))
@@ -1036,7 +1039,6 @@ def run(raw_args):
             warc_files.append(Path(extract_path))
 
     else:
-
         logger.info(f"Running browsertrix-crawler crawl: {cmd_line}")
         crawl = subprocess.run(crawler_args, check=False)
         if (
@@ -1091,7 +1093,7 @@ def run(raw_args):
     logger.info("----------")
     logger.info(
         f"Processing WARC files in/at "
-        f'{" ".join(str(warc_file) for warc_file in warc_files)}'
+        f"{' '.join(str(warc_file) for warc_file in warc_files)}"
     )
     warc2zim_args.extend(str(warc_file) for warc_file in warc_files)
 
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..d51650d
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,14 @@
+import pytest
+
+from zimit import zimit as app
+
+"""
+ cleanup disabled because atexit hooks run at the very end of the Python process
+ shutdown. By the time cleanup() is called, the logging module has already closed its
+ file streams.
+"""
+
+
+@pytest.fixture(autouse=True)
+def disable_zimit_cleanup(monkeypatch):
+    monkeypatch.setattr(app, "cleanup", lambda: None)
diff --git a/tests/data/example-response.warc b/tests/data/example-response.warc
new file mode 100644
index 0000000..143b947
Binary files /dev/null and b/tests/data/example-response.warc differ
diff --git a/tests/test_overwrite.py b/tests/test_overwrite.py
new file mode 100644
index 0000000..e41baca
--- /dev/null
+++ b/tests/test_overwrite.py
@@ -0,0 +1,83 @@
+import pathlib
+
+import pytest
+
+from zimit.zimit import run
+
+TEST_DATA_DIR = pathlib.Path(__file__).parent / "data"
+
+
+def test_overwrite_flag_behaviour(tmp_path):
+    zim_output = "overwrite-test.zim"
+    output_path = tmp_path / zim_output
+
+    # 1st run → creates file
+    result = run(
+        [
+            "--seeds",
+            "https://example.com",
+            "--warcs",
+            str(TEST_DATA_DIR / "example-response.warc"),
+            "--output",
+            str(tmp_path),
+            "--zim-file",
+            zim_output,
+            "--name",
+            "overwrite-test",
+        ]
+    )
+    assert result in (None, 100)
+    assert output_path.exists()
+
+    # 2nd run, no overwrite → should fail
+    with pytest.raises(SystemExit) as exc:
+        run(
+            [
+                "--seeds",
+                "https://example.com",
+                "--warcs",
+                str(TEST_DATA_DIR / "example-response.warc"),
+                "--output",
+                str(tmp_path),
+                "--zim-file",
+                zim_output,
+                "--name",
+                "overwrite-test",
+            ]
+        )
+    assert exc.value.code == 2
+
+    # 2nd run, no overwrite → should fail
+    with pytest.raises(SystemExit) as exc:
+        run(
+            [
+                "--seeds",
+                "https://example.com",
+                "--output",
+                str(tmp_path),
+                "--zim-file",
+                zim_output,
+                "--name",
+                "overwrite-test",
+            ]
+        )
+    assert exc.value.code == 2
+
+    # 3rd run, with overwrite → should succeed
+    result = run(
+        [
+            "--seeds",
+            "https://example.com",
+            "--warcs",
+            str(TEST_DATA_DIR / "example-response.warc"),
+            "--output",
+            str(tmp_path),
+            "--zim-file",
+            zim_output,
+            "--name",
+            "overwrite-test",
+            "--overwrite",
+        ]
+    )
+    assert result in (None, 100)
+    assert output_path.exists()