A locking spring piece for use with a sliding rail includes a connection hole having an inner wall, and a first toothed portion disposed at the inner wall of the connection hole, the first toothed portion comprising at least one tooth, and the at least one tooth of the first toothed portion protruding from the inner wall of the connection hole. The connection hole is configured to allow a connecting member of the sliding rail to pass through, and enable the connecting member to abut against the first toothed portion. A sliding rail assembly, a seat, and a vehicle incorporating the locking spring piece are also provided