This report aims to present a very rare case of progressive limited mouth opening (trismus) in an 8‑year‑old child caused by calcification of the sphenomandibular ligament. This case highlights the difficulty of diagnostic and therapeutic challenges associated with this disorder. We discuss the detailed clinical and radiological approach, including the imaging techniques used to identify the calcification, as well as the treatment option chosen, namely maxillofacial physiotherapy. This report provides new information to the existing literature on this rare entity and to provide useful information for the clinical management of similar cases.