Recently, we proposed a joint transmit/receive frequency-domain minimum mean square error based filtering (joint Tx/Rx MMSE-FD filtering) to improve the transmission performance of broadband single-carrier (SC) multiple-input multiple-output (MIMO) spatial multiplexing. For wireless packet communications, hybrid automatic repeat-request (HARQ) plays an important role as the error control technique to achieve a higher throughput performance. In this paper, we modify and optimize the joint Tx/Rx MMSE-FD filtering for its application to SC-MIMO spatial multiplexing and HARQ packet combining. At each retransmission of the same packet, joint Tx/Rx MMSE-FD filtering is carried out so as to minimize the MSE after packet combining. Computer simulation results show that joint Tx/Rx MMSE-FD filtering offers an improved throughput performance compared to the conventional Rx MMSE-FD filtering.