This paper proposes a foldable stair that is easily deployed for use and folded for storage. It consists of a number of identical deployable scissor-like elements which form the staircases when expanded. In addition to use, the folded stair can be used for hanging clothes and acting as a decoration. The mechanism of the stair is first synthesized in line with the common stairs between two horizontal levels. The actuating mechanism is then synthesized in accordance with two extreme positions of the stair, folded, and unfolded. Because the stair can be folded after use, it is convenient in use and will witness a wide application both indoors and outdoors where there is no enough space for a fixed stair. In addition, this deployable stair is also particularly useful in evacuating people from a building when the disasters such as earthquakes occur.