diff --git a/datasette/cli.py b/datasette/cli.py
index c2c03251..b71275d5 100644
--- a/datasette/cli.py
+++ b/datasette/cli.py
@@ -239,7 +239,12 @@ def package(
         "all IPs and allow access from other machines."
     ),
 )
-@click.option("-p", "--port", default=8001, help="Port for server, defaults to 8001")
+@click.option(
+    "-p",
+    "--port",
+    default=8001,
+    help="Port for server, defaults to 8001. Use -p 0 to automatically assign an available port.",
+)
 @click.option(
     "--debug", is_flag=True, help="Enable debug mode - useful for development"
 )
diff --git a/docs/datasette-serve-help.txt b/docs/datasette-serve-help.txt
index e71e177d..dddade2d 100644
--- a/docs/datasette-serve-help.txt
+++ b/docs/datasette-serve-help.txt
@@ -10,7 +10,8 @@ Options:
                             connections from the local machine will be allowed. Use
                             0.0.0.0 to listen to all IPs and allow access from other
                             machines.
-  -p, --port INTEGER        Port for server, defaults to 8001
+  -p, --port INTEGER        Port for server, defaults to 8001. Use -p 0 to automatically
+                            assign an available port.
   --debug                   Enable debug mode - useful for development
   --reload                  Automatically reload if database or code change detected -
                             useful for development