In-band simultaneous transmit and receive (STR) is expected to increase the efficiency of wireless local area network (WLAN). A major obstacle to implementing STR capability, however, has been insufficient suppression of self-interference. In addition, tackling several problems will require modifications to the current IEEE 802.11 medium access control (MAC) protocol. In this paper, we propose MASTaR, a novel MAC protocol supporting STR in 802.11 WLAN, backward compatible with legacy 802.11. The performance of MASTaR is extensively evaluated via ns-3 simulation. The simulation results demonstrate that significant performance enhancement can be achieved in the current WLANs by an access point (AP) with STR capability and a properly designed MAC protocol.