The average temperature of a red giant star is approximately 3,500 to 5,000 Kelvin (around 3,227 to 4,727 degrees Celsius or 5,841 to 8,741 degrees Fahrenheit). However, it's important to note that there is a range of temperatures within the red giant phase, as these stars can vary in size, luminosity, and evolutionary stage. Larger red giants tend to have lower surface temperatures compared to smaller red giants.