Stimulated Brillouin scattering provides optical gain for efficient and narrow-linewidth lasers in high-Q microresonator systems. However, the thermal dependence of the Brillouin process as well as the microresonator frequencies impose strict temperature control requirements for long term frequency-stable operation. Here, we study Brillouin backaction and use it to both measure and phase-sensitively lock modal temperature to a reference temperature defined by the Brillouin phase matching condition. At a specific lasing wavelength, the reference temperature can be precisely set by adjusting the resonator’s free spectral range. This backaction control method is demonstrated in a chip-based Brillouin laser, but can be applied in all Brillouin laser platforms. It offers a new approach for frequency-stable operation of Brillouin lasers in atomic clock, frequency metrology, and gyroscope applications.