Abstract: A sound velocity sensor for underwater use has an acoustic transmitter and receiver, a path length portion defining an acoustic path and positioned such that a generated acoustic signal propagates along the acoustic path from the acoustic transmitter to the receiver, a temperature sensor in direct contact with the path length portion, and a controller communicatively coupled to these components. The controller is configured to generate the acoustic signal using the acoustic transmitter, determine a transit time of the acoustic signal from the acoustic transmitter to the acoustic receiver, determine a temperature of the path length portion using the temperature sensor, and determine the velocity of the acoustic signal from the transit time and a length of the acoustic path. Determining the velocity includes compensating for a temperature-related change in the length of the acoustic path using the temperature of the path length portion.