FRP-confined concrete-encased steel I-section columns (FCSCs) are an emerging form of hybrid columns. An FCSC consists of an outer FRP tube, an encased steel section and a concrete infill. The FCSCs possess many advantages over conventional reinforced concrete columns, including the excellent corrosion resistance, excellent ductility and ease for construction. Existing studies on FCSCs, however, have been rather limited. This paper presents a combined experimental and theoretical study on the behavior of FCSCs under concentric and eccentric compression. The experimental program included the testing of a total of 14 specimens, with the main variables being the section configuration, the thickness of the FRP tube and the loading scheme. The theoretical part included the development of a model for section analysis based on the so-called fiber element approach. The test results showed that the buckling of steel section was well constrained and the concrete was effectively confined in FCSCs, leading to a very ductile response under both concentric and eccentric compression. The theoretical model was shown to provide reasonably accurate predictions of the test results.