A foldable bicycle handlebar includes a fixing base having two holes on a hinge surface, two handlebar grips each having a joint with a hinge hole, an upper fastener fixed to the joints, two shafts passing through holes of the upper fastener, and a slider fixed to the hinge surface. At least one tenon is on a recess that is on a side of each handlebar grip. The hinge holes, and the two holes of the fixing base, and a gap is between the two joints. The slider has an inner surface, a protrusion is extended outwardly from the inner surface to the gap, and a mortise is formed on the inner surface, so that the tenon is inserted into or removed from the mortise. Whether the handlebar grips are unfolded for use or folded for storage, safety and reliability of the foldable bicycle handlebar are ensured.