A chip stack package includes first and second semiconductor chips. A first redistribution line structure is disposed on a front surface of the first semiconductor chip, and the first redistribution line structure extends onto a side surface of the first semiconductor chip. A second redistribution line structure is disposed on the front surface of the first semiconductor chip, and the second redistribution line structure extends onto the side surface of the first semiconductor chip. A third redistribution line structure is disposed on a front surface of the second semiconductor chip, and the third redistribution line structure extends onto a side surface of the second semiconductor chip to be electrically connected to the second redistribution line structure.