The system (100) for positioning an underwater device (105, 110, 115, 120), comprises: - at least two surface transponders (125, 130, 135) comprising a receiver (160) for receiving radio signals transmitted by a geolocation system (200), - each surface transponder comprising: - a means (180) for estimating at least one radio pseudo-distance, - an attachment (185) to a float, and - a means (140) for communicating information representative of the radio pseudo-distances, and - an underwater acoustic transmitter (140), - the underwater device comprising: - a means (145) for receiving information representative of the radio pseudo-distances, - an acoustic signal receiver (145), - a means (150) for determining one or more acoustic pseudo-distances between at least two underwater acoustic transmitters and the underwater device, and - a means (155) for calculating the position of the device in a terrestrial frame of reference centred on one of the surface transponders.