Twelve concrete columns reinforced longitudinally with fiber-reinforced polymer (FRP) bars were tested under both concentric and eccentric loadings. The investigated parameters were the type of the FRP bar, the longitudinal reinforcement ratio, and the load eccentricity-to-width ratio. The test results showed that the columns reinforced with basalt-FRP (BFRP) and glass-FRP (GFRP) experienced similar load-carrying capacity with a difference of less than 5%. Both types of columns attained lower ultimate capacity than their steel-reinforced counterparts. The contribution of the GFRP and BFRP bars to the ultimate capacity of the columns was similar, approximately 11% of the capacity, as compared to 31% for the steel bars. The effect of increasing the reinforcement ratio on the capacity was more pronounced in the eccentric FRP-reinforced concrete (FRP-RC) columns than the concentric ones. The analytical investigation showed that ignoring the strength contribution of the FRP bars, as recommended by most of the current codes and design guidelines, would result in conservative predictions. It also showed that current Canadian design code recommendation limiting the strains in FRP bars in compression to 2,000 µɛ yielded reasonable predictions of the column capacity.