This digest paper reports on the experimental demonstration of an integrated multimode resonant MEMS temperature sensor by exploiting a thin film ceramic PZT multi-frequency piezoelectric micromachined ultrasonic transducers (pMUTs) in the MHz range. The three resonance modes examined (at 1.107, 3.620, 6.778MHz) all exhibit excellent linearity and responsivity to temperature variations in the range of 0 to 211°C, with temperature coefficients of resonance frequency ($\text{TC}f$) at ∼ −200 to −270ppm/°C for open-loop measurement. We have demonstrated the temperature sensing with real-time tracking of the resonance frequency using a phase-locked loop. We also build a self-sustaining MEMS oscillator and study the closed-loop $\text{TC}f$ which is smaller than the open-loop $\text{TC}f$. We compare the open-loop and closed-loop $\text{TC}f\mathrm{s}$ in both air and vacuum for different type of heaters and find that the $\text{TC}f$ can be affected by heating method, pressure, and measurement scheme.