Treatment of Class III furcation defects involving the supernumerary third roots (radix entomolaris, RE) of mandibular first molars is challenging. A 44-year-old non-smoking Japanese woman presented with discomfort in the mandibular left first molar. A Nabers probe was used to assess furcation involvement, and horizontal probe penetration from the mid-lingual to distolingual region of the furcation was observed. Cone-beam computed tomography revealed a distolingual RE with significant bone destruction around it, indicative of a through-and-through furcation defect. A treatment approach, known as vital root resection, which combined vital pulp therapy and surgical root resection, was implemented. First, deep pulpotomy was performed using mineral trioxide aggregate. Surgical root resection performed 8 weeks later resulted in a marked reduction in the pocket depth and resolution of the furcation defect. Clinical and radiographic evaluations at the 3-year follow-up demonstrated a stable periodontal condition and preserved pulpal vitality, highlighting the effectiveness of this approach. This conservative treatment may be particularly relevant for Asian populations, in whom the prevalence of RE is up to 40%. Although the initial results are promising, further research with larger sample sizes and extended follow-up periods is necessary to establish standardized protocols and evaluate the long-term predictability of this technique.