This study experimentally investigates the behavior of square concrete columns reinforced with glass fiber-reinforced polymer (GFRP) bars under concentric, uniaxial eccentric, biaxial eccentric, and four-point bending loads. The experimental load-moment (P-M) interaction surfaces of GFRP bar-reinforced concrete (GRC) specimens were plotted. Experimental results revealed that the GRC specimens achieved lower maximum axial loads than the corresponding reference steel bar-reinforced concrete (SRC) specimens. The GRC specimens tested under uniaxial and biaxial eccentric loads achieved similar normalized (with respect to concentrically loaded specimens) axial loads, and higher normalized ductility than the SRC specimens. Also, GRC and the SRC specimens achieved similar “knee-shaped” experimental P-M interaction surfaces. Finally, the experimental interaction surfaces are compared with the analytical interaction surfaces developed in this study, and they are found to be in good agreement.