Lead halide perovskites are promising semiconductors for high-performance photonic devices. Because the refractive index determines the optimal design and performance limit of the semiconductor devices, the refractive index and its change upon external modulations are the most critical properties for advanced photonic applications. Here, we report that the refractive index of halide perovskite CH₃NH₃PbCl₃ shows a distinct decrease with increasing temperature, i.e., a large negative thermo-optic coefficient, which is opposite to those of conventional inorganic semiconductors. By using this negative coefficient, we demonstrate the compensation of thermally induced optical phase shifts occurring in conventional semiconductors. Furthermore, we observe a large and slow refractive index change in CH₃NH₃PbCl₃ during photoirradiation and clarify its origin to be a very low thermal conductivity supported by theoretical analysis. The giant thermo-optic response of CH₃NH₃PbCl₃ facilitates efficient phase modulation of visible light.