This letter presents a method for obtaining an exact average symbol error rate (ASER) of M-phase shift keying (M-PSK) transmission for the N^th best opportunistic amplify-and-forward (OAF) relay systems over Rayleigh fading channels. This approach begins with deriving the relay selection probability when a relay is selected as the N^th best one with respect to the received signal-to-noise ratio. We then derive the modified moment generating function (MGF) for the N^th best OAF relay systems by taking the given N^th best-relay selection probability into consideration. Based on the modified MGF, we derive the exact ASER which accurately explicates the N^th best OAF relay system characteristics. Simulation results confirm the exactness of the analysis results for M-PSK transmission with respect to the number of relays, the N^th best relay selection, and the relay position.