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We report a case of transient unilateral exotropia, a rare complication that occurred after an infraorbital local anesthetic injection, and present the possible mechanism underlying this adverse effect. A 64-year-old man underwent an excisional biopsy under local anesthesia for bilateral lower eyelid masses. He received a subcutaneous injection of a local anesthetic (a combination of 1:100,000 epinephrine and 2% lidocaine). Intraoperatively, we observed a homogeneous, fat-attenuated mass with thin fibrous septae inside the orbital septum of each eye. Following excision of the masses, the patient developed severe diplopia accompanied by exotropia, impaired adduction, and mydriasis of the right eye. The pupillary light reflex, visual acuity, and visual field test results were within normal ranges. Notably, his symptoms completely resolved 4 hours postoperatively without intervention. To our knowledge, transient unilateral exotropia following infraorbital local anesthetic injection is rare. The exotropia was attributed to diffusion of the epinephrine/lidocaine solution, which may have affected the ciliary ganglion and the medial rectus muscle. Caution is warranted during local anesthetic injections in patients who undergo periorbital surgery.