A foldable leg support contains two rotary arms and a supporting surface between the two rotary arms, and the two rotary arms are rotatably connected with a seat member. When in a rest position, the foldable leg support is pulled outward to expand the supporting surface. Hence, the legs are capable of being extended on the supporting surface comfortably to improve blood circulation, and the foldable leg support is suitable for any size of the legs. When the foldable leg support is not used, the supporting surface is put between the two rotary arms, and, then, the rotary arms and the supporting plane surface are folded under an office chair, saving storage space.