Disclosed are devices for attaching a seat to a stand-up paddleboard. The devices generally have an open “C”-shape that is designed and configured to accommodate the side rail of a stand-up paddleboard while maintaining flush contact with at least a portion of the bottom of the stand-up paddleboard. The present disclosure also relates to a system for attaching a seat to a stand-up paddleboard. The system includes the above-described device and a mechanism for connecting the device to a seat disposed on the stand-up paddleboard. The mechanism for connecting the device to a seat can be, for example, a strap or a hook. The connecting device can be varied to accommodate the location on the seat to which the connecting device is attached.