FRP-confined concrete-encased cross-shaped steel columns (FCCSCs) are an emerging form of hybrid columns, consisting of a square FRP outer tube, a cross-shaped steel section and a concrete infill. The steel section in FCCSCs serves as ductile longitudinal reinforcement, and its buckling is well constrained by the concrete and the FRP tube. In addition, the steel section provides confinement to the concrete together with the FRP tube, so that the concrete in FCCSCs is effectively confined despite their square cross section. This paper presents a systematic experimental study on FCCSCs. The test variables included the flange width, flange thickness and web thickness of the steel section, and the thickness of FRP tube. It is shown that the various steel sections in the tested FCCSCs all provided substantial confinement to the concrete, while the effect of the steel confinement is less pronounced for FCCSCs with a thicker FRP tube. The test results show that the variation of the cross-sectional dimensions of the steel section, within a reasonably wide practical range, has marginal effects on the behaviour of the concrete in FCCSCs. In addition, a simple method has been proposed to predict the axial load capacity and ultimate axial strain of FCCSCs.