<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We propose an accurate, robust avalanche photodiode (APD) receiver optimization scheme for multiple-input/multiple-output (MIMO) free-space optical links with <emphasis><formula formulatype="inline"><tex>$Q$</tex></formula></emphasis>-ary pulse position modulation in the presence of atmospheric turbulence. Through extensive error probability <emphasis><formula formulatype="inline"><tex>$(P_{e})$</tex> </formula></emphasis> analysis, we show that selecting the APD gain such that the nonfading receiver-array signal-to-noise ratio is maximized essentially minimizes <emphasis><formula formulatype="inline"><tex>$P_{e}$</tex></formula></emphasis> in turbulent MIMO channels with <emphasis><formula formulatype="inline"><tex>$M = 2$</tex></formula></emphasis> transmitters and <emphasis><formula formulatype="inline"> <tex>$N = 2$</tex></formula></emphasis> receivers, for several turbulence and background-radiation levels of interest. </para>