The success of endodontic treatment depends on a correct understanding of root canal anatomy, identification of anatomical variations, and correct canal positioning. Mandibular molars, which usually have two roots, may harbor an additional root, such as the radix entomolaris (RE), incompatible with this expected anatomy. The location of this variant root, especially close to the distolingual root, can complicate the endodontic procedure. This case series provides important insights into the clinical management of RE variation based on three cases with RE. The clinical evaluation, diagnosis, and root canal treatment procedures are presented. The teeth were asymptomatic at follow-up examinations, and radiographic examinations revealed normal/healthy periapical status. Endodontic specialists and endodontic treating physicians must take a careful and conscious approach to the presence of additional roots in mandibular molars during root canal treatment processes. Proper understanding and handling anatomical variations such as RE is a critical factor in the success of root canal treatment.