Interval velocities for marine sediments are generally obtained from source-receiver separations at various offsets. A method is described for estimating interval velocities using a monostatic configuration. Testing is performed using simulated data from rough layered seabeds and interval velocity is estimated within less than 1% of the true value. Monostatic multibeam data from the Gulf of Lion are also presented which exhibit many characteristics similar to the simulated data. The method applied to the measured data yield an interval velocity of 1569 m/s in an 18 m sediment layer. This accords with nearby independent data from cores and wide-angle reflection analysis.