This paper reports on an investigation of the flexural behavior and serviceability performance of long-span square concrete members with a shear span-to-effective depth ratio (a/d) greater than 5, internally reinforced with uniformly distributed fiber-reinforced polymer (FRP) bars. The study comprised testing of four large-scale square members up to failure under four-point bending. The specimens measured 400 mm (16 in.) in width, 400 mm (16 in.) in depth, and 6000 mm (236.22 in.) in length. The test parameters included the longitudinal reinforcement ratio and the longitudinal reinforcement type, including glass FRP (GFRP), carbon FRP (CFRP), and steel bars. Test results show that the deformability of the tested FRP-reinforced concrete (FRPRC) specimens ranged between 7.0 and 10.4, which significantly exceeds the requirements of North American codes. Moreover, the nominal flexural strength of the specimen reinforced with GFRP bars was 1.9 times that of the steel counterpart specimen when the reinforcement ratios were similar. An analytical strain-compatibility model capable of predicting the flexural strength of the tested specimens was developed and compared to the experimental results. In addition, the measured crack widths and deflections were analyzed and compared at service load conditions to those predicted using models in the literature as well as in design guidelines and codes. The effect of uniformly distributed bars on the flexural strength and serviceability of square FRPRC members was also investigated, revealing that the presence of side bars significantly enhanced the serviceability performance in terms of crack width and deflection. On the other hand, the contribution of these side bars to flexural strength was minimal.