The invention provides a vehicle steering system intended to dispose a vehicle steering mechanism efficiently. The vehicle has a front wheel and a rear wheel, further comprises a steering conversion mechanism for converting a rotary input to an input shaft (33) into an arbitrary amount of rotation and/or an arbitrary turning force and outputting it from an output shaft (34); a steering handlebar coupled to the input shaft; a support portion (4a) coupled to the output shaft and supporting the front wheel of the vehicle; and a housing (31) accommodating the steering conversion mechanism, wherein the steering conversion mechanism is mounted at the front end of a body frame (2) of the vehicle integrally therewith through the housing. The steering conversion mechanism includes at least one of a variable ratio steering mechanism (32b) for causing an input steering angle to the steering handlebar and an output steering angle to the front wheel to differ from each other at an arbitrary ratio, and a power assist mechanism (32a) for supplying an arbitrary supplementary power to an input operation to the steering handlebar.