A computational theory for determining electron transfer rate constants is formulated based on an instanton expression for the quantum rate and the self-consistent solution of the imaginary time nonadiabatic steepest descent approximation. The theory obtains the correct asymptotic behavior for the electron transfer rate constant in the nonadiabatic and adiabatic cases, and it smoothly bridges between those two limits for intermediate couplings. Furthermore, no assumptions regarding the form of the diabatic potentials are invoked (e.g., harmonic) and more than two diabatic states can be included in the calculations. The method thereby holds considerable promise for computing electron transfer rate constants in realistic condensed phase systems.