Replace the plug.

Yes. That’s the permanent fix, but try tightening first—sometimes that’s all it needs.

Most cooler drain plugs are cheap plastic with a rubber gasket. Over time the gasket flattens, hardens, or just gives up. Tightening might help for a while, but the real solution is a new plug. They’re universal and cost a few bucks. If you can’t find the exact model, a rubber stopper from the hardware store works too.

If the plug is threaded and the leak is minor, wrap the threads with plumber’s tape (Teflon tape) before screwing it in. That adds a seal and buys you time. But if it’s really leaking, the gasket is shot.

A leaking cooler is just a wet mess waiting to happen.