A fiber‐optic sensor for continuous temperature measurement from room temperature to ≳700 °C is presented. The device is based upon the excitation of the synthetic crystal alexandrite with light from a diode laser operating at 670 nm and the measurement of its fluorescence lifetime. A simple low cost optical configuration is described together with the introduction of a new effective signal processing scheme for fluorescence lifetime measurement, and a high sensitivity and repeatability are observed, at best ±1 °C.