The aim of this article was to report a case of a mandibular second premolar with four root canals. Mandibular premolars have earned the reputation for having aberrant anatomy. A thorough knowledge of the basic root canal anatomy and its variations is necessary for successful root canal treatment. The incidence of four root canals in mandibular second premolars has only been documented in the literature by a few case reports. This clinical article describes the successful root canal treatment of a mandibular second premolar with four root canals with the aid of the operating microscope. Keywords: anatomical variation, endodontic treatment, mandibular second premolar, operating microscope