A temperature sensor was demonstrated and fabricated by coating thermosensitive film around a fiber coupler. Based on the multicladding equivalent method, the coated fiber coupler was simplified to a conventional one. With the high thermo-optical coefficient of organic–inorganic solgel material, a good sensing result was achieved. The range of temperature measured is from $-50°\mathrm{C}$ to $100°\mathrm{C}$ . The resonant wavelength has a shift of about $25\mathrm{nm}$ . A sensitivity of $0.17\mathrm{nm}/°\mathrm{C}$ is achieved. With the advantages of having a simple structure and being unaffected by the instability of the light source, the proposed fiber coupler temperature sensor will find wide applications.