The study describes a case of congenital restrictive strabismus with ipsilateral enophthalmos caused by a supernumerary extraocular muscle (SEOM). Orbital MRI and OCT of the posterior segment of the eye confirmed the posterior origin of the SEOM, its traction on the eyeball, and hypoplasia of the middle and lateral rectus muscles of the eye. These findings explain the mechanistic basis of restricted eye motility and enophthalmos. Genetic analysis revealed a pathogenic duplication in the 22q11.21 region, which is the first reported association with restrictive strabismus caused by SEOM and extends the phenotype of the 22q11.2 duplication syndrome. Surgical treatment is risky because of the deep location of SEOM near the optic nerve and hypoplasia of the muscle, so conservative management is recommended. The case highlights the importance of MRI in the diagnosis of anomalous orbital structures.