With the advent of gallium nitride (GaN) as an enabling material system for the solid-state lighting industry, high-power and high-brightness light-emitting diodes (LEDs) with wavelengths ranging from near ultraviolet to blue are being manufactured as part of a tremendously large and ever-increasing market. However, device self-heating and the environment temperature significantly deteriorate the LED's optical performance. Hence, it is important to accurately quantify the LED's temperature and correlate its impact on optical performance. In this work, three different characterization methods and thermal simulation were used to measure and calculate the temperature rise of an InGaN/GaN LED, as a result of self-heating. Nanoparticle-assisted Raman thermometry was used to measure the LED mesa surface temperature. A transient Raman thermometry technique was utilized to investigate the transient thermal response of the LED. It was found that under a 300 mW input power condition, self-heating is negligible for an input current pulse width of 1 ms or less. The temperature measured using nanoparticle-assisted Raman thermometry was compared with data obtained by using the forward voltage method (FVM) and infrared (IR) thermal microscopy. The IR and Raman measurement results were in close agreement whereas the data obtained from the widely accepted FVM underestimated the LED temperature by 5–10%. It was also observed that an increase in environment temperature from 25 °C to 100 °C would degrade the LED optical power output by 12%.