Glass fiber-reinforced polymer (GFRP) bars are a valid alternative to traditional steel bars for reinforcing concrete structures in specific situations. Although the elastic modulus of GFRP bars is generally 20–25% of that of steel bars, which entails for a higher cross-sectional area of GFRP than of steel bars for the same axial stiffness, they have superior durability with respect to the steel counterpart, which makes them suitable for applications in aggressive environments, such as those exposed to the chloride attack. Among these, RC bridge decks represent a typical case where GFRP bars strongly increases the structure durability, since they do not suffer for the chloride attack caused by the use of deicing agents. Bridge decks are subjected to cyclic loads that may induce fatigue issues. However, quite limited studies are available in the literature regarding the fatigue behavior of GFRP bars. In this paper, the test method for tensile fatigue provided by the European standard ISO 10406–1 is discussed and modified to account for the actual cyclic stresses applied to representative RC members subjected to fatigue, namely bridge decks. The results of a tensile test of a GFRP bar according to the modified procedure are then presented and discussed.