diff --git a/docs/environment-variables.md b/docs/environment-variables.md
index a6624365..ac20fc52 100644
--- a/docs/environment-variables.md
+++ b/docs/environment-variables.md
@@ -2311,7 +2311,7 @@ How long to keep cached search results before they expire.
 | `QBITTORRENT_DOWNLOAD_DIR_BOOK` | Server-side directory where book torrents are downloaded (optional, uses qBittorrent default if not specified) | string | _none_ |
 | `QBITTORRENT_DOWNLOAD_DIR_AUDIOBOOK` | Server-side directory where audiobook torrents are downloaded (optional, uses qBittorrent default if not specified) | string | _none_ |
 | `QBITTORRENT_TAG` | Tag(s) to assign to qBittorrent downloads. Leave empty for no tags. | string (comma-separated) | _empty list_ |
-| `TRANSMISSION_URL` | URL of your Transmission instance (use https:// for TLS) | string | _none_ |
+| `TRANSMISSION_URL` | URL of your Transmission instance (use https:// for TLS). Without a port, https uses 443 and http uses 9091, so add :9091 to an https URL served on 9091 | string | _none_ |
 | `TRANSMISSION_USERNAME` | Transmission RPC username (if authentication enabled) | string | _none_ |
 | `TRANSMISSION_PASSWORD` | Transmission RPC password | string (secret) | _none_ |
 | `TRANSMISSION_CATEGORY` | Label to assign to book downloads in Transmission | string | `books` |
@@ -2516,7 +2516,7 @@ Tag(s) to assign to qBittorrent downloads. Leave empty for no tags.
 
 **Transmission URL**
 
-URL of your Transmission instance (use https:// for TLS)
+URL of your Transmission instance (use https:// for TLS). Without a port, https uses 443 and http uses 9091, so add :9091 to an https URL served on 9091
 
 - **Type:** string
 - **Default:** _none_
diff --git a/shelfmark/download/clients/settings.py b/shelfmark/download/clients/settings.py
index 67a62a7a..35b9dfb8 100644
--- a/shelfmark/download/clients/settings.py
+++ b/shelfmark/download/clients/settings.py
@@ -815,7 +815,10 @@ def prowlarr_clients_settings() -> list[SettingsField]:
         TextField(
             key="TRANSMISSION_URL",
             label="Transmission URL",
-            description="URL of your Transmission instance (use https:// for TLS)",
+            description=(
+                "URL of your Transmission instance (use https:// for TLS). Without a port, "
+                "https uses 443 and http uses 9091, so add :9091 to an https URL served on 9091"
+            ),
             placeholder="http://transmission:9091",
             show_when={"field": "PROWLARR_TORRENT_CLIENT", "value": "transmission"},
         ),
diff --git a/shelfmark/download/clients/torrent_utils.py b/shelfmark/download/clients/torrent_utils.py
index a1b270d4..d72c1a9a 100644
--- a/shelfmark/download/clients/torrent_utils.py
+++ b/shelfmark/download/clients/torrent_utils.py
@@ -382,7 +382,9 @@ def parse_transmission_url(url: str) -> tuple[str, str, int, str]:
     if protocol not in ("http", "https"):
         protocol = "http"
     host = parsed.hostname or "localhost"
-    port = parsed.port or 9091
+    # An omitted port is scheme-specific: HTTPS behind a reverse proxy is 443,
+    # while HTTP (including the empty-URL default) stays on Transmission's RPC port.
+    port = parsed.port if parsed.port is not None else (443 if protocol == "https" else 9091)
     path = parsed.path or "/transmission/rpc"
 
     # Ensure path ends with /rpc
diff --git a/tests/prowlarr/test_torrent_utils.py b/tests/prowlarr/test_torrent_utils.py
index 8d9a3be7..985019df 100644
--- a/tests/prowlarr/test_torrent_utils.py
+++ b/tests/prowlarr/test_torrent_utils.py
@@ -87,6 +87,14 @@ class TestParseTransmissionUrl:
         assert port == 9091
         assert path == "/transmission/rpc"
 
+    def test_parse_https_url_without_port_uses_443(self):
+        """Test an HTTPS URL with no port uses the implicit HTTPS port."""
+        protocol, host, port, path = parse_transmission_url("https://transmission.myserver.org")
+        assert protocol == "https"
+        assert host == "transmission.myserver.org"
+        assert port == 443
+        assert path == "/transmission/rpc"
+
     def test_parse_url_with_ip_address(self):
         """Test parsing URL with IP address."""
         protocol, host, port, path = parse_transmission_url("http://192.168.1.100:9091")