— Despite significant research efforts, closed-form bit error rate (BER) expressions for multiple-input multiple-output (MIMO) systems employing transmit antenna selection and orthogonal space-time block codes (OSTBCs) are not available. We thus derive exact closed-form expressions for the BER of Graycoded M-ary one and two-dimensional amplitude modulations when an OSTBC is employed and N transmit antennas out of total Lt antennas are selected for transmission. We also derive tight closed-form approximate BER for M-PSK constellations. Our BER expressions are valid for a frequency-flat Rayleigh fading MIMO channel and can be evaluated without numerical integration methods.