In reinforced concrete (RC) columns, the efficiency of confinement that is provided by fiber–reinforced polymer (FRP) composites is usually limited by the buckling of longitudinal steel bars. This situation is even more critical in noncircular columns in which the FRP ruptures due to a combination of buckling of the steel bars and FRP stress concentrations at the corners. The main objective of this study is to observe the effect of internal steel bars on the axial compressive response of noncircular RC column specimens confined with polyethylene terephthalate (PET) FRP, which possesses a large rupture strain (LRS) capacity. In total, 32 specimens were tested under monotonic axial compression. The parameters considered are the number of PET FRP layers, the stirrup spacing, and the cross-sectional aspect ratio. The test results indicate that the strength and deformation capacities of the confined specimens were significantly influenced by the parameters that were considered. Of interest, with an increase in PET FRP confinement, the effect of buckling length became less significant. In addition, the test results indicate that the PET FRP confinement provided a weak but prolonged constraint on the buckling of longitudinal bars. Moreover, PET FRP efficiently sustained the stress concentrations of bar buckling at corners that led to a considerable ductile response before failure.