Multiuser multiple-input and multiple-output (MU-MIMO) technology is regarded as one of the most promising technologies to cope with the enormous amount of network traffic in wireless local area networks. However, combining uplink and downlink MU-MIMO on the basis of a distributed coordination function causes serious channel resource loss because of collisions between uplink and downlink transmissions. In this paper, we propose a phase-divided medium access control (PD-MAC) protocol, in which a novel integrated frame structure is introduced that allocates two phases for uplink and downlink in a periodic cycle to remove contentions between uplink and downlink transmissions. PD-MAC also adopts an overhead mitigation scheme to reduce control overhead in user selection and employs an algorithm to balance uplink and downlink throughputs. An analytical model is developed to characterize the PD-MAC network throughput. Simulation results show that PD-MAC outperforms the previous unified MAC with regard to network throughput by 21.2%, and that the overhead mitigation scheme improves PD-MAC network throughput by 47.3%.