diff --git a/README.md b/README.md
index d096f5a..ba20b2c 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ command line.
 - Upload videos from a directory using YAML sidecars for metadata and thumbnails.
 - Schedule, start, stop, and update YouTube livestream broadcasts, including RTMP ingest details.
 - Multi-channel profiles: manage several channels from one machine and switch per command.
-- Comments moderation: list, approve, reject, and ban from the CLI.
+- Comments moderation: list, reply, approve, reject, and ban from the CLI.
 - Channel analytics queries via the YouTube Analytics API.
 - Playlists: bulk-add by search and reorder by views with one command.
 
diff --git a/docs/api-quota.md b/docs/api-quota.md
index 9b9b909..34d69a1 100644
--- a/docs/api-quota.md
+++ b/docs/api-quota.md
@@ -28,6 +28,7 @@ not eat into the table below.
 | `search.list`                                | 100 units          |
 | `videos.update`                              | 50 units           |
 | `comments.setModerationStatus`               | 50 units           |
+| `comments.insert` (reply)                    | 50 units           |
 | `playlists.insert` / `update` / `delete`     | 50 units           |
 | `playlistItems.insert` / `update` / `delete` | 50 units           |
 | `liveBroadcasts.insert`                      | 50 units           |
diff --git a/docs/comments.md b/docs/comments.md
index 185d933..c4444ca 100644
--- a/docs/comments.md
+++ b/docs/comments.md
@@ -40,7 +40,20 @@ ytstudio comments reject <comment-id> --ban     # also ban the author
 `reject` hides the comment from public view. Pass `--ban` to also ban the
 author from the channel.
 
+## Reply
+
+```bash
+ytstudio comments reply <comment-id> --text "Thanks for watching!"
+```
+
+`reply` posts a public reply to a comment and prints the new reply id.
+Replies are flat on YouTube: `<comment-id>` must be a **top-level** comment
+id (the `id` shown by `comments list`), not the id of another reply. Passing
+a reply id (or an otherwise invalid id) returns a clear error. Like
+`publish` and `reject`, `reply` executes immediately.
+
 !!! note "Quota"
 
-    `comments.setModerationStatus` costs about 50 quota units per call. For
-    larger moderation runs, see [API quota](api-quota.md).
+    `comments.setModerationStatus` and `comments.insert` (reply) each cost
+    about 50 quota units per call. For larger moderation runs, see
+    [API quota](api-quota.md).
diff --git a/src/ytstudio/commands/comments.py b/src/ytstudio/commands/comments.py
index a29e7b5..8db0d63 100644
--- a/src/ytstudio/commands/comments.py
+++ b/src/ytstudio/commands/comments.py
@@ -174,3 +174,34 @@ def reject(
     """Reject comments (hide from public display)"""
     count = _set_moderation_status(comment_ids, "rejected", ban_author=ban)
     console.print(f"{count} comment(s) rejected")
+
+
+@app.command()
+def reply(
+    comment_id: str = typer.Argument(
+        help="Top-level comment ID to reply to (from 'comments list')"
+    ),
+    text: str = typer.Option(..., "--text", "-t", help="Reply text"),
+):
+    """Reply to a top-level comment.
+
+    Replies are flat on YouTube: COMMENT_ID must be a top-level comment id
+    (the id shown by 'comments list'), not the id of another reply.
+    """
+    service = get_data_service()
+    try:
+        response = api(
+            service.comments().insert(
+                part="snippet",
+                body={"snippet": {"parentId": comment_id, "textOriginal": text}},
+            )
+        )
+    except HttpError as e:
+        if e.resp.status in (400, 404):
+            console.print(
+                f"[red]Could not reply to '{comment_id}'.[/red] Pass a top-level "
+                "comment id from 'comments list' (not a reply id)."
+            )
+            raise typer.Exit(1) from None
+        handle_api_error(e)
+    console.print(f"Reply posted: {response['id']}")
diff --git a/tests/conftest.py b/tests/conftest.py
index c43ef39..bea085e 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -152,6 +152,13 @@ def create_mock_service():
     }
     service.commentThreads.return_value.list.return_value = comments_list
 
+    comments_insert = MagicMock()
+    comments_insert.execute.return_value = {
+        "id": "UgwReply789",
+        "snippet": {"parentId": MOCK_COMMENT["id"], "textOriginal": "Thanks for watching!"},
+    }
+    service.comments.return_value.insert.return_value = comments_insert
+
     search_list = MagicMock()
     search_list.execute.return_value = {
         "items": [{"id": {"videoId": MOCK_VIDEO["id"]}}],
diff --git a/tests/test_comments.py b/tests/test_comments.py
index 059e793..4c7d1d7 100644
--- a/tests/test_comments.py
+++ b/tests/test_comments.py
@@ -1,5 +1,7 @@
 from datetime import UTC, datetime, timedelta
+from unittest.mock import MagicMock
 
+from googleapiclient.errors import HttpError
 from typer.testing import CliRunner
 
 from ytstudio.main import app
@@ -47,3 +49,28 @@ def test_list_comments_disabled(self, mock_auth):
         )
         result = runner.invoke(app, ["comments", "list", "--video", "test_video_123"])
         assert result.exit_code == 1
+
+    def test_reply(self, mock_auth):
+        result = runner.invoke(
+            app, ["comments", "reply", "UgwComment123", "--text", "Thanks for watching!"]
+        )
+        assert result.exit_code == 0
+        assert "UgwReply789" in result.stdout
+        insert = mock_auth.comments.return_value.insert
+        body = insert.call_args.kwargs["body"]
+        assert body["snippet"]["parentId"] == "UgwComment123"
+        assert body["snippet"]["textOriginal"] == "Thanks for watching!"
+
+    def test_reply_requires_text(self, mock_auth):
+        result = runner.invoke(app, ["comments", "reply", "UgwComment123"])
+        assert result.exit_code != 0
+
+    def test_reply_invalid_parent(self, mock_auth):
+        resp = MagicMock()
+        resp.status = 400
+        mock_auth.comments.return_value.insert.return_value.execute.side_effect = HttpError(
+            resp, b'{"error": {"message": "invalid"}}'
+        )
+        result = runner.invoke(app, ["comments", "reply", "not_a_top_level_id", "--text", "hi"])
+        assert result.exit_code == 1
+        assert "top-level comment id" in result.stdout