Background <p>Mandibular first molars typically exhibit complex root and canal anatomy. A thorough understanding of their morphological variations is crucial for endodontists to achieve successful root canal treatment. A C-shaped root canal system is a unique anatomical variation characterized by a C-shaped or semi-circular cross-sectional morphology, observed predominantly in mandibular second molars. The prevalence rate of the C-shaped root canal system is approximately 2.7–48.7% in the mandibular second molar, while the condition is rarely seen in mandibular first molars, particularly in bilateral cases.</p> Case presentation <p>This report details an uncommon case of a C-shaped root canal system in bilateral mandibular first molars: the right molar exhibited a fused root with two separate mesial and two distal canals, while the left molar displayed a single oval mesial canal and a semicolon-shaped distal canal. To our knowledge, this is the first report of the presence of four separate canals within a fused C-shaped root in the mandibular first molar.</p> Conclusions <p>While C-shaped root variations are detected in mandibular first molars, a thorough knowledge of normal root canal anatomy and associated variations presents a significant challenge for clinicians in terms of successful endodontic treatment.</p>