In this study, 2.4 mm thick high-strength martensitic steel plates with a tensile strength of 1500 MPa were friction stir welded at various welding speeds of 40, 60, 80, 100, 120 mm/min and a constant rotation speed of 300 rpm. Sound joints could be obtained when the welding speed was 40, 60 and 80 mm/min, while a kissing bond was found in the joint welded at 100 and 120 mm/min. It was revealed that the peak temperature exceeded AC3 (the end temperature at which all ferrite transformed to austenite when the steel was heated) for all the welding conditions and martensitic structures were finally formed in the stir zone of the joints. A significant decrease in hardness was located in the heat-affected zone, which had a transitional microstructure from tempered martensite near base metal to a mixed structure containing hard martensite, soft ferrite and bainite near stir zone. For the sound joints, the specimen was fractured in the heat-affected zone during tensile tests and the highest tensile strength could reach about 1058 MPa.