The combination of glass fiber–reinforced polymer (GFRP) and ultrahigh-performance concrete (UHPC) to form structural members has generated significant interest due to their excellent durability and mechanical properties. This paper presents the flexural behavior and design methodology of GFRP-reinforced UHPC beams. Eight reinforced UHPC beams were tested to failure, varying in longitudinal reinforcement type (steel and GFRP), flexural reinforcement ratio, and steel fiber volume fraction (1% and 2%). Two flexural failure modes, including crack localization followed by rupture of GFRP (tension failure) and progressive crushing of UHPC followed by rupture of GFRP (compression failure), were observed in the tested GFRP-reinforced beams. Substitution of steel bars with GFRP bars resulted in delayed crack localization and a significant improvement in flexural strength by 54.9% and ultimate displacement by 55.7%. Increasing the GFRP reinforcement ratio showed a trend of increased flexural capacity, ultimate deformation, and energy dissipation capacity. Increasing the steel fiber volume in UHPC improved the flexural capacity of the tension failure–controlled beam, but had a slight effect on the flexural capacity of the compression failure–controlled beam. In addition, two different models were used to calculate beam deflection, and were compared with experimental results at the service load levels. Considering the fiber-bridging mechanism, a flexural strength model for GFRP-reinforced UHPC beams was developed. Finally, a minimum reinforcement ratio was proposed to ensure progressive failure of GFRP-reinforced UHPC beams.