Abstract Glass fiber reinforced polymer (GFRP) bars represent a valid solution as internal reinforcement of concrete members for some particular applications. GFRP reinforcing bars (rebars) have a high-strength-to-weight ratio and good resistance to corrosion. However, they may suffer of degradation when exposed to specific aggressive environments and when subjected to long-term sustained stress. To increase their durability, design guidelines available in the literature limit the stress level in the rebar. However, such limitations are based on few experimental results and represent conservative estimations of the bar long-term behavior. In this paper, the results of 9 short-term tensile tests and 17 long-term tensile tests on GFRP bars are presented. The long-term tests included relaxation and creep tests for 1000 and 2000 h considering five different initial applied stress levels. The results obtained are described by two new relaxation and creep functions able to reproduce the bar behavior from the application of the initial applied stress. The functions proposed allow for obtaining the long-term relaxation losses of the reinforcing bars for different stress levels.