Combining seawater, recycled coarse aggregate (RCA), and glass fiber reinforced polymer (GFRP) reinforcement in concrete is potentially advantageous from a sustainability perspective. This paper reports on the results of an experimental study on the short-term flexural performance of seawater-mixed recycled-aggregate concrete beams with GFRP bars. Twelve medium-scale reinforced concrete (RC) beams (150×260×2200 mm) were tested under four-point loading. The test variables included the mixing water (seawater/freshwater), aggregate type (conventional/recycled), and reinforcement material (black steel/GFRP). A wide range of flexural properties, including failure mode, cracking behavior, load-carrying capacity, deformation, energy absorption, and ductility were characterized and compared among the beam specimens. The results suggest that the use of seawater and RCA in concrete has insignificant effects on the flexural capacity of RC beams, especially if concrete strength is preserved by adjusting the mixture design. Altering reinforcement material had a strong influence on the flexural capacity and performance of the tested specimens: the GFRP-RC beams exhibited higher load-carrying capacities (on average 25%) but inferior deformational characteristics as compared to their steel-reinforced counterparts. Theoretical predictions were obtained for the flexural capacity, crack width, and deflection of steel- and GFRP-RC beams based on their corresponding design guides, and compared with the experimental results.