In standard blepharoplasty, the orbital septum is opened commonly to enter the preaponeurotic space. This is to anchor the leading edge of the levator palpebral fascia to the skin of a proposed supratarsal fold and to remove supraorbital fat. Incision of the orbital septum is involved occasionally with accidental injury to an inconstant arterial branch at its lateral side, which causes severe bleeding. Anatomic knowledge of the vessel on the orbital septum is conducive to preventing accidental arterial injury. The authors encountered a relatively large artery running vertically at the lateral side of the orbital septum in 50 eyelids of 230 blepharoplasty patients (460 eyelids). They investigated the branch by injecting red latex into the ophthalmic arteries in 20 hemidissected faces of fresh cadavers. At the lateral aspect of the orbital septum, a branch of the superior lateral palpebral artery, which we named the lateral septoaponeurotic artery (LASA), was confirmed in five of 13 specimens. The LASA originates from the superior lateral palpebral artery just after it divides into peripheral and marginal arcades. The LASA pierces the levator aponeurosis and orbital septum at the upper level of the tarsal plate. It anastomoses with a branch of the supraorbital artery at the superior aspect of the orbit. The authors think severe bleeding occurs when the LASA is injured during blepharoplasties. Cautious treatment of the lateral septoaponeurotic vessel minimizes undesirable complications.