Congenital masseter muscle hypertrophy (CMMH) is a developmental disorder characterized by unilateral muscle's overgrowth, resulting in facial asymmetry and dento-maxillary disorders. Although the disease is usually diagnosed by clinical examination, paraclinical investigations such as Computer Tomography (CT), Magnetic Resonance (MR) or ultrasounds (US) are advanced diagnostic methods. This article is the case report of a CMMH in a young male adult, presenting the clinical aspects and using a new diagnostic approach based on US investigation of the masseter muscle.