Dynamic temperature monitoring at critical locations of IGBT modules is a key means to improve the reliability of high-power converters. However, most existing thermal model-based methods suffer from temperature estimation errors due to model parameter variations and loss calculation errors. To address this problem, based on the reduced-order thermal model, an H∞ observer-based robust 3-D thermal monitoring method for IGBT modules is proposed in this paper. Through the optimized design of the observer feedback gain, the thermal model and real-time temperature information are effectively combined, which reduces the temperature estimation error in the worst case. Thus, the proposed method is more robust to model parameter uncertainty and loss error than the conventional temperature observers. Experiment validations of the proposed H∞ observer and conventional observers are provided. The results demonstrate that the proposed observer achieves the highest temperature estimation accuracy under various system uncertainties, making it an effective solution for reliable online thermal monitoring of IGBT modules over the whole life cycle.