A board-to-board connector includes a first circuit board and a second circuit board. The first circuit board has at least one cut. The second circuit board includes a tongue assembly disposed corresponding to the cut, a plurality of connection portions disposed on two opposite surfaces of the tongue assembly, and a plurality of conductive portions electrically connected to the first circuit board. Therefore, the board-to-board connector can effectively reduce the distance between the first circuit board and the second circuit board, which is applicable to the increasingly lightweight and compact electronic devices.