Abstract: A security device is provided for a foldable chair and includes a control member movably mounted between a pivot member and a vertical section of a base member, an elongated second slot defined in the control member and communicating with a first slot of the pivot member, a linking bar mounted between the two pivot members and having two distal ends each of which has a reduced diameter smaller than that of the linking bar to extend through the first slot of the pivot member and securely engage with a corresponding control member such that the control member is able move with the linking bar synchronously, the linking bar being restricted to slide along the first slot of the pivot member between a first position where the control member aligns with a first end of the pivot member such that the pivot member together with the control member are pivotally engaged with the base member, and a second position where the control member is moved to a position beyond the first end of the pivot member such that the pivot