Energy efficiency plays a significant role for the network lifetime extension in wireless sensor networks (WSNs). On the other hand, higher priority information requires Quality of Service (QoS) to ensure their delivery with minimized delay. Furthermore, WSN nodes are low powered devices powered by a small battery where the capacity is limited. Therefore, there is a need to establish an energy-efficient Medium Access Control (MAC) protocol with minimum packet delays. In this paper, we propose an adaptive energy-efficient synchronous Quality of Service (QoS) MAC protocol, called Timeout Multi-priority based MAC (TMPQ-MAC) protocol. TMPQ-MAC proposed an adaptive energy-efficient MAC protocol which can dynamically extend and end the active time to handle the load variations in the network. The TMPQ-MAC also considers four different packet priorities so that data packets with urgent information get transmitted with minimum delay. Finally, the performance of the protocol is evaluated through simulations and provided a comparison with Multi-priority QoS MAC (MPQ-MAC), QoS Aware Energy-Efficient MAC (QAEE-MAC) and Energy Harvested Receiver Initiated MAC (ERI-MAC) protocols. The simulation results show that the TMPQ-MAC improves the energy consumption at the sender nodes and the average packet delays by up to 7.8 and 7.9, respectively over other protocols. © 2021 IEEE.