To provide an adult/child convertible chair which can be converted from adult use to child use, or vice versa, with one-touch operation, without using a tool, at any time.

A side frame attached to a support frame for fixing legs is provided with pivotal portions that pivots an adult-use backrest turnably/fixably within a plane perpendicular to the chair width direction, and is disposed with a locking mechanism between the lower surface of an adult-use seat and the support frame. When the adult-use seat is lifted and integrally turned with the backrest, the adult-use seat is positioned to be a child-use backrest and the adult-use backrest is positioned to be a child-use seat, and the locking mechanism is engaged/fixed. After the locking mechanism is unlocked and the child-use chair is operated reversely, the child-use chair can be converted into the adult-use chair again.