Specimens of epoxy resin reinforced with ultrahigh-modulus polyethylene (UHMPE) fibers were immersed in water, and their swelling characteristics were recorded at various temperatures. In addition to an estimation of the response of those composite, the above study aimed at the exploration of the role of the fiber-matrix interface on the water sorption. Therefore, specimens containing original, calendered, and corona-treated fibers were tested. UHMPE fibers were found to limit the extent of sorption due to the nonhydrophilic character of polyethylene. However, specimens with poor interfacial properties, such as those with the original, untreated fibers, showed enhanced sorption since their surface area is drastically increased. As expected, the raise of temperature has a positive contribution to water sorption, and, furthermore, it seemed to affect the interface between epoxy and calendered fibers. On the other hand, the increase of filler volume fraction leads to a decrease in the amount of water uptake. The water transport in the neat epoxy resin specimens is rather diffusion-controlled, and this behavior was also recorded for the composite specimens reinforced with original UHMPE, which presented the maximum absorption.