Carbon fibre reinforced polymer (CFRP) has been used in strengthening different types of steel members in bending and compression. However there is still a lack of understanding on the behaviour of CFRP reinforced steel beams subject to torsion. This paper presents a theoretical approach to investigate CFRP strengthening of thin-walled steel square hollow section (SHS) subjected to torsion. Various CFRP strengthening configurations including vertical, spiral and reverse-spiral wrapping are considered. Laminate composite mechanics is introduced to incorporate the contribution of CFRP strengthening to the torsional stiffness and strength of the SHS. By introducing reasonable failure criteria, the torsional behaviour of CFRP strengthened SHS can be predicted corresponding to different failure modes, i.e. CFRP shear, CFRP fracture, CFRP buckling, steel yielding. The relationship between torsion and twist angle have also been identified. The theoretical solutions are validated with lab testing results and reasonable agreement has been achieved.