Concrete-filled steel tubular (CFST) columns exhibit enhanced axial compressive strength because of lateral confinement offered by the steel tube and its interaction with the concrete core. The axial load-deformation behaviour of a CFST column can be significantly influenced by the presence of an interface gap between the steel tube and infill concrete core, which may occur due to various factors. This paper aims to numerically investigate the influence of interface gap on the axial load versus axial deformation response of square-section CFST (SCFST) stub columns across various cross-sectional sizes. To assess the impact of interface gap on different-sized SCFST stub columns, the square-section side length of SCFST columns has been increased from 140 to 200 mm with an increment of 20 mm, and each column section has been analysed for no gap, and 1 and 2 mm interface gap all around. ABAQUS is employed to perform a non-linear finite element analysis on SCFST columns, where the infill concrete core and the steel tube are discretized using the C3D8R element. The plastic behaviour of the infill concrete core is modelled using the Extended Drucker-Prager (E-DP) approach. The present study demonstrates that while an interface gap has a minimal effect on the SCFST column's axial compressive strength, it plays a crucial impact on the column's ductility and lateral bulging.