A theory for linear surface gravity waves on a semi-infinite layer of viscoelastic fluid described by a Jeffrey model is presented. Results are given for the decay rate and the phase velocity as a function of the parameters of the fluid: a nondimensional time constant, and a ratio of the retardation time to the relaxation time. At small wave numbers the behavior is Newtonian. In other cases depending on the nondimensional parameters, a number of possible other behaviors exist. The influence of the non-dimensional parameters on the growth rate of Rayleigh-Taylor instability is also discussed.