Glass fibre-reinforced polymer (GFRP) bars present multiple advantages over conventional steel reinforcement including high strength-to-weight ratios and corrosion resistance. Whereas many studies are reported in the literature on the behaviour of GFRP-reinforced concrete members, the performance of GFRP-prestressed concrete beams has been the focus of only a few research works. Zawam et al. (Zawam M, Soudki K, West JS (2017) Effect of prestressing level on the time-dependent behaviour of GFRP prestressed concrete beams. J Composites Constr 21(4) (2017)) demonstrated the potential of GFRP for prestressing applications with satisfactory creep performance under sustained loading. Very limited information available regarding the unloading behaviour of concrete beams after being subjected to sustained loading. Such information would be beneficial for example in cases of concrete bridges during rehabilitation projects. Similarly, very few studies are available that describe the behaviour of GFRP-prestressed concrete beams. This study addresses both of these gaps through an investigation of the unloading behaviour of GFRP-prestressed/reinforced concrete beams after being subjected to different levels of sustained loading for a period of 300 days.