This paper reports on a piezoelectrically actuated and sensed flexural-mode microelectromechanical (MEMS) resonant device with electrothermally and piezoelectrically tunable resonant frequency. The device is designed as a multilayer circular membrane (diaphragm) resonator with a lead-zirconium-titanate piezoelectric actuator and sensor as well as platinum electrothermal tuning electrodes placed on the top of a silicon-carbide diaphragm. The design enables active electrothermal frequency tuning independent of the piezoelectric input/output operation of the device. The performance of the fabricated device has been tested using two-port transmission frequency response measurements that are performed at atmospheric conditions. Electrothermal tuning and piezoelectric tuning introduce the unique feature of shifting the resonant frequency downward and upward, respectively. The resonant frequency has been tuned by applying DC voltages in the range 0 V – 5 V. The measurements have shown that an 886 kHz device exhibits a frequency tuning range of about –8,400 ppm when tuned electrothermally, and a tuning range of about +2,400 ppm when tuned piezoelectrically. Simulated results have shown that the wider frequency range for electrothermal tuning is a result of larger change in induced stress in the diaphragm for a given DC voltage, as well as the fact that the electrothermal tuning mechanism effectively serves to relax the residual tensile stress in the diaphragm.