The quadrature component unbalance generated by a non-ideal component such as an imperfect 90-degree phase shifter is an inevitable physical phenomenon and leads to performance degradation in a practical coherent M-ary phase shift keying (MPSK) transceiver. In this letter, we present an exact and general expression involving the one- and two-dimensional Gaussian Q-functions for the symbol error rate (SER) of MPSK with I/Q phase unbalance over an additive white Gaussian noise (AWGN) channel. The SER expression provided here offers a convenient way to evaluate the performance of MPSK systems for various cases of practical interest.