Objective: Present a clinical case detailing the evaluation and treatment of an idiopathic obturator internus strain in a collegiate tennis player. Background: Isolated injury to the obturator internus muscle is uncommon. In cases that do present, patients often report often report a mechanism for injury involving falling or kicking. In previously reported cases, diagnosis is confirmed by magnetic resonance imaging. Treatment: Patient presented to the athletic training clinic complaining of left hip external rotator tightness and pain, and was unable to recall a mechanism of injury. Following the failure of conservative treatment, final diagnosis of a grade II obturator internus strain was made via magnetic resonance imaging. Using the confirmed diagnosis, the athletic training staff was able to progress strengthening and range of motion exercises until the patient returned to competition six weeks post injury. Conclusion: Obturator internus strains are possible causes of posterior hip pain and tightness. Currently, there are no reliable evaluation techniques for the obturator internus that do not involve diagnostic imaging. Further research needs to be conducted to determine if there are any signs and symptoms specific to obturator internus injuries.