We demonstrate real-time semiconductor processing temperature control based on direct, noninvasive, in situ measurement of wafer temperature by diffuse reflectance spectroscopy. The precision of the controlled temperature was maintained within +/-0.5 degrees C with an update time of 2 s over a temperature range from 25 to 600 degrees C for semi-insulating GaAs wafers. Direct control of the wafer temperature using diffuse reflectance spectroscopy is compared to the usual situation where the wafer heater temperature is controlled. The direct control of wafer temperature using diffuse reflectance spectroscopy is shown to be a significant improvement over conventional heater control technology in accuracy, stability, repeatability and response rate, for dynamic control of wafer temperature in the range of 25-600 degrees C. Our technology can also be applied to Si and other semiconductor wafers following appropriate calibration.