Biological complications involving dental implants include peri-implant diseases such as peri-implant mucositis and peri-implantitis. The latter presents with progressive bone loss from the alveolar crest in a coronal apical direction. However, a separate disease entity termed retrograde peri-implantitis (RPI), which presents with progressive bone loss at the periapex of the implant, also exists and may be of particular interest to endodontists because it typically presents with periapical pathology of both the implant and adjacent tooth or at a site that previously housed an endodontically treated tooth. The reported prevalence of retrograde peri-implantitis is 0.26%, which is much lower than the prevalence of marginal peri-implantitis; however, its incidence increases to 7.8% when teeth adjacent to the implant exhibit an endodontic infection. It is positively correlated with a shorter distance between the implant and the adjacent tooth and a shorter time elapsed from the endodontically treated adjacent tooth to implant placement. This case report describes a patient diagnosed with an RPI lesion (RPI) associated with an adjacent endodontically treated tooth with a persistent periapical radiolucent lesion. The diagnosis, possible etiology, and management of the RPI lesion is thoroughly reported including follow-up visits showing complete resolution after subsequent periodontal and endodontic therapy. Endodontic evaluation of teeth adjacent to the implant site should be performed for primary prevention of RPI. Proper classification of RPI will aid in determining the course of treatment; class 1 and 2 cases require endodontic therapy of the involved teeth for healing to occur.