A novel simple noncoherent sequence estimation (NSE) metric for M-ary differential phase-shift keying (MDPSK) transmission over intersymbol interference (ISI) channels is proposed. In contrast to previously reported NSE schemes, the phase reference symbol required for metric calculation can be obtained efficiently in a recursive manner. Simulations demonstrate the high performance of the novel scheme, which can approach the performance of coherent maximum-likelihood sequence estimation (MLSE) while the complexity of metric computation remains relatively low.