A vehicle body side structure is provided. A vehicle body side structure includes a front fender, a side sill garnish, and an aerodynamic duct. The front fender has a fender recess portion below a first wheel arch portion. The side sill garnish has a second wheel arch portion that faces the fender recess portion at the outer side in the vehicle width direction at interval and faces a rear end of the first wheel arch portion. The aerodynamic duct is interposed between the second wheel arch portion and the fender recess portion. The width of the second wheel arch portion gradually decreases in the front-rear direction of the vehicle body as it extends upward from the front end portion of the side sill garnish to an upper edge.