Exact average symbol error probability (SEP) of the general order rectangular quadrature amplitude modulation (QAM) with maximum ratio combining (MRC) diversity over L independent Nakagami-m fading channels with arbitrary fading parameters is derived in this paper. The obtained exact expression is presented in terms of the Lauricella multiple hypergeometric functions, which can be evaluated numerically using its integral or converging series representation. The new result can be used to study the impact of diversity reception on the performance of general rectangular QAM. Finally, it is shown that the derived expression reduces to some specific cases previously published.