A high-sensitivity temperature sensor based on the harmonic Vernier effect is proposed and verified by experiments. The main component of the sensor is a Sagnac interferometer consisting of two sections of polarization maintaining fibers (PMFs) spliced with an intersection angle of 45° between their fast axes. The harmonic Vernier effect is achieved by setting the length of one of the PMFs an integral multiple (i-times) of the length of the other plus a detuning factor. Compared with the Sagnac interferometer based on the fundamental Vernier effect, the temperature sensitivity of the harmonic Vernier effect is higher, reaching i+1 times of that of the fundamental Vernier effect (i is the order of the harmonic).