A temperature sensor based on a switchable dual-wavelength fiber Bragg grating (FBG) laser with a semiconductor saturable absorber mirror (SESAM) is presented and demonstrated experimentally. The repetition rate of Q-switched pulses is ~17 kHz. The results demonstrate that the measured temperature has good linearity to the wavelength spacing of the two lasing wavelengths and has a temperature sensitivity of 21 pm/oC covering a range of -10—22 °C. The experimental results prove the feasibility of the proposed temperature sensor.