Invertebrates that use a fluid-filled cavity surrounded by contractile tissue (a hydrostatic skeleton) propel themselves efficiently underwater and penetrate regions inaccessible to legged or wheeled devices. A hydrostatic robot would therefore be invaluable for exploring marine environments. We have constructed a three-segment hydrostatic robot that locomotes underwater. Each segment consists of two solid circular disks connected by four equidistant shape memory alloy springs. A fluid-filled bladder in the center of each segment provides hydrostatic skeletal support. The 15.5 cm long robot moves at speeds up to 0.6 cm/s, can turn up to 21°, and can change its length by up to 16%. A model of the robot’s kinematics has also been developed.