Glass fiber-reinforced polymer (GFRP) bars are an attractive solution to reinforce concrete members exposed to aggressive environments. The use of GFRP reinforcing bars in concrete bridge slabs, which can be exposed to the salt attack due to the use of de-icing salts, potentially increases the reinforced concrete (RC) member durability. However, RC bridge slabs are subjected to cyclic loading that may induce fatigue degradation and eventual failure of the member. In this paper, the fatigue behavior of GFRP reinforcing bars is studied using tensile-tensile cyclic tests. The specimen is comprised of a thermoset GFRP bar with a 12.5 mm nominal diameter. Steel pipes are bonded to the specimen ends to prevent anchorage failure. A constant stress amplitude is considered, whereas the mean stress is varied to investigate the bar fatigue life with respect to this parameter. The results obtained provide fundamental indications on the stress amplitude and mean stress that can be applied to the GFRP bars to obtain a certain fatigue behavior.