Background Mobile medical applications (Apps) offer innovative solutions for patients’ self-monitoring and new patient management opportunities. Prior to routine clinical application feasibility and acceptance of disease surveillance using an App that includes electronic (e) patient-reported outcome measures (PROMs) warrant evaluation. Therefore, we performed a proof-of-concept study in which rheumatoid arthritis (RA) patients used an App (RheumaLive) to document their disease. Methods Accurate PROM reporting via an App in comparison to paper-based versions was investigated to exclude media bias. Sixty participants recruited from 268 consecutive RA outpatients completed paper-based and electronic PROMs (Hannover Functional Questionnaire/derived HAQ; modified RA disease activity index) using the App at baseline and follow-up visits. Between visits, patients used their App on their own smartphone according to their preferences. The equivalence of PROM data and user experiences from patients and physicians were evaluated. Results Patients’ (78.3% female) mean (SD) age was 50.1 (13.1) years, disease duration 10.5 (9.1) years, and paper-based HAQ 0.78 (0.59). Mean confidence in Apps scored 3.5 (1.1, Likert scale 1 to 6). ePROMs’ scores obtained by patients’ data entry in the App were equivalent to paper-based ones and preferred by the patients. After 3 months, the App retention rate was 71.7%. Patients' overall satisfaction with the App was 2.2 (0.9, Likert scale 1 to 6). Patients and physicians valued the App, i.e., for patient-physician interaction: 87% reported that it was easier for them to document the course of the disease using the App than “only” answering questions about their current health during routine outpatient visits. Further App use was recommended in 77.3% of the patients, and according to physicians, in seven patients, the App use contributed to an increased adherence to therapy. Conclusion Our study provides an essential basis for the broader implementation of medical Apps in routine care. We demonstrated the feasibility and acceptance of disease surveillance using a smartphone App in RA. App use was convincing as a reliable option to perform continuous, remote monitoring of disease activity and treatment efficacy. Trial registration ClinicalTrials.gov, NCT02565225. Registered on September 16, 2015 (retrospectively registered).