The longitudinal dispersion of contaminants due to mass transport induced by random surface waves is examined. The wave kinematics under random waves are first derived, including the second-order drift profile. The relationship between the dispersive coefficient and wave drift is then established following the approach in J. Hydraul. Res. (2000) 38:41–48. Subsequently, the magnitude of the longitudinal dispersive coefficient is examined under both developed and developing sea states, represented by the P–M spectrum and the Wen spectrum, respectively. The results show that the longitudinal dispersion by random waves is significantly stronger than an equivalent regular sea state with a similar wave energy density. Furthermore, the dispersive effect in the developing sea state is found to be stronger than the developed sea state with comparable wave-energy density, which can be attributed to the differences in the spectral shape between the two states.