A hub cap is provided for boat trailer wheel hubs that includes an air valve. The valve may take several different forms. A hub cap employs a security air valve that is inserted through an aperture formed within the hub cap body. The valved hub cap allows the hub cavity to be temporarily pressurized when a boat trailer is partially submerged during the launch and recovery of a boat. Additional features may be incorporated in the hub cap, including a pressure release valve, which prevents damage to the hub seal, an O-ring groove in the hub cap body, which seals the cap to the hub and may also secure the hub cap to the wheel hub body. A separate spring retainer clip is also provided, which secures the hub cap to the hub body.