The diode junction temperature (Tj) of light emitting devices is a key parameter affecting the efficiency, output power, and reliability. Herein, we present experimental measurements of the Tj on ultraviolet (UV) AlGaN nanowire (NW) light emitting diodes (LEDs), grown on a thin metal-film and silicon substrate using the diode forward voltage and electroluminescence peak-shift methods. The forward-voltage vs temperature curves show temperature coefficient dVF/dT values of −6.3 mV/°C and −5.2 mV/°C, respectively. The significantly smaller Tj of ∼61 °C is measured for the sample on the metal substrate, as compared to that of the sample on silicon (∼105 °C), at 50 mA, which results from the better electrical-to-optical energy conversion and the absence of the thermally insulating SiNx at the NWs/silicon interface. In contrast to the reported higher Tj values for AlGaN planar LEDs exhibiting low lateral and vertical heat dissipation, we obtained a relatively lower Tj at similar values of injection current. Lower temperatures are also achieved using an Infrared camera, confirming that the Tj reaches higher values than the overall device temperature. Furthermore, the heat source density is simulated and compared to experimental data. This work provides insight into addressing the high junction temperature limitations in light-emitters, by using a highly conductive thin metal substrate, and it aims to realize UV AlGaN NWs for high power and reliable emitting devices.