diff --git a/custodian_cli.py b/custodian_cli.py
index 962edba..4a9dd38 100644
--- a/custodian_cli.py
+++ b/custodian_cli.py
@@ -97,13 +97,24 @@ def cmd_register(args: argparse.Namespace) -> None:
         print(f"ERROR: Unknown domain '{domain}'. Valid: {', '.join(valid_domains)}")
         sys.exit(1)
 
-    # ── Step 3: Topic ID lookup ────────────────────────────────────────────────
+    # ── Step 3: Topic ID lookup (auto-create if new domain) ───────────────────
     print(f"==> Looking up topic for domain '{domain}' ...")
     topics = _api_get("/topics/?status=active")
     match = next((t for t in topics if t.get("domain_slug") == domain), None)
     if not match:
-        print(f"ERROR: No active topic found for domain '{domain}'.")
-        sys.exit(1)
+        print(f"    No topic found — creating one for domain '{domain}' ...")
+        slug = re.sub(r"[^a-z0-9]+", "-", domain.lower()).strip("-")
+        try:
+            match = _api_post("/topics/", {
+                "slug": slug,
+                "title": project_name,
+                "domain": domain,
+                "status": "active",
+            })
+            print(f"    Topic created: {match['title']} ({match['id']})")
+        except Exception as e:
+            print(f"ERROR: Could not create topic for domain '{domain}': {e}")
+            sys.exit(1)
     topic_id = match["id"]
     print(f"    topic_id: {topic_id}")