A floor brace fastening structure includes: a floor brace that is configured to extend toward a vehicle lower side from an instrument panel reinforcement provided at a front portion of a passenger compartment; a vehicle floor to which the floor brace is fastened using a fastener; and a reinforcing member that is formed of a material with a higher coefficient of extension than a material that forms the floor brace, the reinforcing member being attached to the floor brace and being fastened to the vehicle floor using the fastener.