AbstractA concrete‐filled steel tube (CFST) specimen was tested under uniaxial loading to acquire the nonlinear monotonic and cyclic response of the steel tube in CFST members including inelastic cyclic local buckling. The failure mode, hysteretic response, envelope curves, strength, and stiffness degradation of the CFST specimen were comprehensively studied. The observed failure modes of the CFST specimen were local buckling and ductile fracture of the steel tube due to ultra‐low‐cycle fatigue. The effective stress‐strain curve of the steel tube exhibited negative stiffness due to local buckling. The compressive resistance of the CFST specimen decreased significantly when local buckling progressed at the steel tube. The tensile unloading stiffness slightly decreased relative to the elastic stiffness of the CFST specimen; conversely the compressive unloading stiffness experienced severe deterioration upon increased cyclic loading. The tensile stiffness decreased gradually during the whole loading process, while the post‐capping stiffness remained stable.