A oneway valve for an automatically inflatable air cushion includes a valve body having an air inlet port and an air outlet port, a valve seat mounted in the valve body and formed with a plurality of conducting holes connected to the air inlet port and the air outlet port, and a valve stem movably mounted in the air inlet port and movable to seal the conducting holes. Thus, the oneway valve prevents the air from leaking from the air cushion during the inflation process and prevents the air from entering the air cushion during the deflation process, so that the air cushion is inflated and deflated completely and exactly.