The torsional behavior of solid reinforced concrete (RC) members reinforced with fiber-reinforced polymer (FRP) bars has been the subject of several experimental studies. No experimental research, however, seems to have focused on RC box girders reinforced with FRP bars under a pure torsional moment. This paper reports the results of an experimental investigation on the torsional strength and behavior of full-scale RC box girders reinforced with longitudinal glass FRP (GFRP) bars. All specimens measured 380 mm (15 in.) in height, 380 mm (15 in.) in width, 100 mm (4 in.) wall thickness, and 4000 mm (157.48 in.) in length. They were tested under pure torsional loading over a clear span of 2000 mm (78.74 in.). The test specimens consisted of four RC box girders with longitudinal GFRP bars and one RC box girder with longitudinal steel bars as a reference. All the specimens were constructed without web reinforcement to study the contribution of the longitudinal reinforcement to torsional strength. The test variables included the longitudinal reinforcement ratio (ranging between 1.10 and 2.74%) and the type of longitudinal reinforcement (GFRP or steel). The test results indicate that increasing the GFRP longitudinal reinforcement ratio increased the torsional strength afterthe initiation of the first diagonal crack, especially for specimens with a high reinforcement ratio. In addition, theoretical torsional moment-twist curves were developed and gave predictions consistent with the experimental test results. Lastly, the ultimate torsional strength of the GFRP-RC box girders without web reinforcement was estimated with the CSA S806-12 (R2017) design equation with a modification related to the GFRP tensile strain limit.