A foldable chair includes a chair frame and a seat frame. The chair frame includes two spaced apart front legs, two spaced apart rear legs pivotally connected to the two front legs respectively, and a supporting member transversely extended between the two rear legs and having a bottom inclined engaging side. The chair frame is adapted to pivotally fold between an unfolded position and a folded position. The seat frame includes two seat arms pivotally connected to the front legs respectively, a seat platform provided between the two seat arms, and a retention arm which is transversely extended between two seat arms and has a corresponding upper inclined retention side.