Stainless steel tubular beam sections have been increasingly utilised for structural applications due to the enhanced properties of stainless steel over conventional mild steel. However, thin-walled stainless steel sections are not effectively used for several practical applications due to the higher costs and its response to easily undergo local buckling under flexural loading. Consequently, to overcome these effects, concrete infilling is a generally preferred strengthening mechanism that enhances the overall capacity and makes the sections economic. Earlier research studies on the flexural behaviour of various concrete-infilled stainless steel composite beam specimens are highly limited. Therefore, this study mainly focuses on the flexural behaviour of bare stainless steel tubular beams, sugarcane bagasse ash blended concrete-infilled stainless steel composite beam, and fibre-reinforced sugarcane bagasse ash blended concrete-infilled stainless steel composite beam. The study uses sugarcane bagasse ash as a pozzolan in the infill concrete to produce durable and sustainable concrete. In the same manner, crimped steel fibres are incorporated to enhance the ductility of the beam specimen. The parameters such as ultimate strength, maximum flexural deformation, initial stiffness, failure modes, ductility, and moment-rotation profiles were comprehensively investigated for all the beam specimens. Furthermore, numerical modelling was performed for the fibre-reinforced sugarcane bagasse ash blended concrete-infilled stainless steel composite beam specimen and was validated with the experimental results. It was witnessed that adding infill concrete to the stainless steel tube enhanced the beam specimens' load-carrying capacity, stiffness and ductility.