One aspect of the present disclosure provides a seatback fastening structure for fastening a seatback to a body of a vehicle. A body bracket includes a main body portion covered by a seat bracket. The main body portion includes a receiving surface, an upper surface extending from a vertical upper end of the receiving surface, and a first lateral surface and a second lateral surface extending from both ends of the receiving surface in a width direction. The seat bracket includes a base portion covering the receiving surface, a retaining portion retaining the upper surface from vertically above, a first side wall and a second side wall respectively abutting on the first lateral surface and the second lateral surface. The main body portion and the base portion are fastened with a bolt.