An underbody structure of a vehicle includes a cross member, a side sill, and a gusset. The cross member is provided on a floor panel and extends in a vehicle width direction. The side sill is coupled to an outer end of the floor panel in the vehicle width direction and extends in a front-rear direction. The cross member and the side sill are separated in the vehicle width direction. The gusset is interposed between them. The gusset includes front and rear side walls, an upper wall, a protrusion, and front and rear flanges. Each side wall includes a lower end coupled to an outer end of the cross member in the vehicle width direction. The upper wall is coupled to an upper end of each side wall. The protrusion is provided at an outer end of the upper wall in the vehicle width direction.