In the context of two-wave with diffuse power (TWDP) fading scenarios, this letter analyzes the average symbol error probability (SEP) of rectangular quadrature amplitude modulation (QAM) for single- and multi-channel receptions. Specifically, novel exact expressions are derived for the average SEP in nondiversity and maximal-ratio combining (MRC) schemes. Our derived SEP results apply to general operating scenarios with distinct TWDP fading parameters and arbitrary number of diversity branches.