The world unlimited Water Speed Record is the officially recognised fastest speed achieved by a water-borne vehicle, irrespective of propulsion method. The current unlimited record is 511.11 km/h (317.59 mph), achieved by Australian Ken Warby in the Spirit of Australia in 1978.

The record is one of the sporting world's most hazardous competitions. Of the thirteen people who have attempted the record since June 1930, seven have died. There have been two official attempts to beat Ken Warby's 1978 record, and both resulted in the death of the pilot; Lee Taylor (1980) and Craig Arfons (1989). Despite this, there are several teams currently working to make further attempts.

The record is ratified by the Union Internationale Motonautique (UIM).

Ken Warby (born 9 May 1939) is an Australian motorboat racer, who currently holds the water speed record of 275.97 knots (511.10 km/h; 317.58 mph), set on Blowering Dam on 8 October 1978.As a child, Warby's hero was Donald Campbell, who died attempting to break the record in 1967.