A distributed sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (FBGs) was designed and fabricated for temperature measurements in commercial boilers under field conditions. Three cascaded FBGs with Bragg wavelengths ranging from 1540 nm to 1570 nm were inscribed in a commercial air-clad single-crystal sapphire fiber, which were packaged inside high purity alumina sheath tubes. The packaged sensor was deployed in a commercial boiler through an existing view port during operation. The tests were performed in a coal-fired boiler, whose temperature varied based on power demands in the local area. The reflected signal from the sensor was simultaneously recorded by an onsite optical interrogation system and converted into temperature information, which was monitored remotely in real time through the Virginia Tech network. The performance of the sensor was consistent during the entire test duration of 42 days, over the course of which it measured temperatures up to 700°C, showing the survivability of the sensing system in a field environment. The system has a demonstrated temperature measurement range from 25°C to 1200°C.