The measurement of turbine inlet temperature is challenging because of high temperatures and complicated physical access, but continuous measurement of the turbine inlet temperature is very important for maximizing turbine efficiency and increasing durability. This paper provides in-situ turbine rotor inlet temperature (TRIT) measurements in an 8200 kW operating gas turbine engine. The measurements were obtained using integrated spectral infrared (ISIR) emission from the water vapor of the combustion gases entering the turbine rotor. The method utilizes a sapphire optical fiber to convey the signal from the turbine wall to outside the turbine casing. All components are capable of long-term exposure to the turbine operating conditions. The temperature measurements were obtained at 6 operating conditions between 50% and full load. The TRIT temperature was also determined using more than 20 test cell inputs and Solar Turbine’s commercial test cell engine model. The two temperatures (measured and modeled) were within 11 K (less than 1%) across the load sweep. Uncertainty calculations suggest that the uncertainty of the measurement can be expected to be ±2.9% within a confidence interval of 95%. The method also yields the nozzle guide vane surface temperature which was found to increase monotonically with increasing load.