Background: Endometriosis is a multifactorial benign disorder characterized by the abnormal presence of endometrial tissue in an extraendometrial site. Although extrapelvic endometriosis is uncommon, symptomatic involvement of the kidney is exceedingly rare. This benign disease can mimic several urologic processes, but because of its scarcity in clinical practice, it is seldom considered in the differential. Case Presentation: In this report, we describe the case of a 45-year-old woman with flank pain and hematuria, who was found to have a left renal mass on cross-sectional imaging. After robotic partial nephrectomy, pathologic analysis revealed an endometrial implant within the renal parenchyma. Conclusion: This case of renal endometriosis highlights how this benign disease process can mimic several more sinister urologic processes.