A seat support structure for facilitating assembly of a frame support member. The seat support structure includes a seat support portion mounted on a seat rail of a vehicle, the seat support portion having an opening portion; a mount rubber having an engaging groove portion on an outer peripheral portion thereof, an insertion portion arranged closer to a distal end side than the engaging groove portion, and a through hole penetrating through the mount rubber. Upon mounting the seat, the insertion portion of the mount rubber is inserted into the opening portion of the seat support portion, the portion of the seat to be supported is inserted into the through hole, and the mount rubber is moved in the radial direction in an inside of the opening portion, and a portion of a groove wall of the engaging groove portion is brought into pressure contact with the opening portion.