Purpose Clinical trials are critical to informing cancer care but often are hampered by slow accrual and lack of generalizability because of poor geographic accessibility. We tested the feasibility of replacing onsite study visits with telemedicine visits in a prospective clinical trial. Methods Castration-näive patients with prostate cancer and a rising serum prostate-specific antigen after definitive local therapy were eligible. Patients were required to have a single onsite visit for enrollment. Study treatment consisted of oral metformin 850 mg daily for 1 month followed by 850 mg twice daily for 5 months. Telehealth video visits (televisits) were conducted monthly by using a Health Insurance Portability and Accountability Act-compliant smartphone application. The primary objective was to determine the feasibility of telemedicine-enabled study visits. Secondary objectives were defining safety, anticancer activity, quality of life, and patient satisfaction. Results Fifteen patients with a median age of 68 years (range, 57 to 83 years) and median one-way driving time to the study center of 71 minutes (range, 12 to 147 minutes) were enrolled. The patients completed 84 eligible televisits (completion rate, 100%; 95% CI, 0.80 to 1). Diarrhea was the most common adverse event but was limited to grade 1 in severity; a single patient experienced grade ≥ 3 adverse events. Seven patients (46.7%; 95% CI, 24.8% to 69.9%) had a ≤ 20% increase in prostate-specific antigen relative to baseline. Patients agreed or strongly agreed that they would participate in a telemedicine-enabled clinical trial in the future. Conclusion To our knowledge, this interventional oncology clinical trial is the first to be conducted through telemedicine. Telemedicine-enabled trials are feasible and may overcome geographic barriers to trial participation. Metformin was generally well tolerated but associated with modest anticancer activity.