Accessory cusps are occasional variations in tooth morphology that can be detected clinically. Third molars have a wide range of eruption patterns and shapes but are commonly impacted. Unlike root morphology, variations in the crown morphology of the third molar have rarely been reported. Variations in crown morphology are important because they can affect clinical outcomes. Here, we present a rare case of fully erupted mandibular third molar (MTM) with multiple accessory cusps. This case report describes a case of a 31-year-old female Kenyah with a unique crown morphology of her fully erupted, vital, and functional mandibular third molar (MTM) which has multiple accessory cusps seen on the buccal surface, giving the appearance of a double tooth. Her MTM presented with an incipient caries lesion, which was managed conservatively. Documenting variations in tooth morphology is crucial for individual identification. While most people opt to have their MTMs extracted, early management of well-erupted and functional teeth should be undertaken to prevent disease progression.