The influence of the fiber/matrix interface of pultruded glass fiber-reinforced polymer (GFRP) composites exposed to hygrothermal environments, including deionized water immersion and saltwater immersion at 20 +/- 5 degrees C, 30 +/- 1 degrees C, and 60 +/- 1 degrees C for 180days, was investigated. The effect of moisture absorption on tensile properties was discussed. After 180days of immersion in deionized water, tensile strength and modulus of specimens decreased 25.7% and 26%, whereas the equivalent respective losses were 2.1% and 18.2% for specimens immersed in saltwater. The short-beam-shear test and the single-fiber fragmentation test were selected to reflect the degradation of macro- and micro-interfacial properties, respectively. After 180-day immersion in deionized water and saltwater at 60 +/- 1 degrees C, the inter-laminar shear strength of specimens decreased 28.8% and 18.5%, respectively, and the corresponding interfacial shear strength decreased 53.2% and 23.5%, indicating that the diffusion rate of micro-interface was higher than that of macro-interface in the fiber direction. Immersion in all media leads to pronounced degradation in tensile strength, modulus, and inter-laminar shear strength. Furthermore, based on the change of interfacial strength and Weibull distribution, a prediction model was proposed to describe the degradation trends and temperature effects on ultimate bearing capacity of pultruded GFRP composites in hygrothermal environments.