Importance In the age of information and patient-centered care, new methods of delivering postoperative care must be developed and evaluated. Objective To determine whether follow-up care delivered via a mobile app can be used to avert in-person follow-up care visits compared with conventional, in-person follow-up care in the first 30 days following ambulatory surgery. Design, Setting, and Participants A randomized clinical trial was conducted from February 1 to August 31, 2015, among ambulatory patients undergoing breast reconstruction at an academic ambulatory care hospital. Patients were randomly assigned to receive follow-up care via a mobile app or at an in-person visit during the first 30 days after the operation. Analysis was intention-to-treat. Main Outcomes and Measures The primary end point was the number of in-person follow-up visits during the first 30 days after the operation. Secondary end points were the number of telephone calls and emails to health care professionals, patient-reported convenience and satisfaction scores, and rates of complications. Results Of the 65 women in the study (mean [SD] age, 47.7 [13.4] years), 32 (49%) were in the mobile app group, and 33 (51%) were in the in-person follow-up care group. Those in the mobile app group attended a mean of 0.66 in-person visits, vs 1.64 in-person visits in the in-person follow-up care group, for a difference of 0.40 times fewer in-person visits (95% CI, 0.24-0.66; P < .001) and sent more emails to their health care professionals during the first 30 days after the operation (mean, 0.65 vs 0.15; incidence rate ratio, 4.13; 95% CI, 1.55-10.99; P = .005) than did patients in the in-person follow-up care group. This statistically significant difference was maintained at 3 months postoperatively. The mobile app group reported higher convenience scores than the in-person follow-up care group (incidence rate ratio, 1.39; 95% CI, 1.09-1.77; P = .008). There was no difference between groups in the number of telephone communications, satisfaction scores, or complication rates. Conclusions and Relevance Patients undergoing ambulatory breast reconstruction can use follow-up care via a mobile app to avert in-person follow-up visits during the first 30 days after the operation. Mobile app follow-up care affects neither complication rates nor patient-reported satisfaction scores, but it improves patient-reported convenience scores. Trial Registration clinicaltrials.gov Identifier: NCT02318953.