In a community-based study of 100 patients with rheumatoid arthritis, a smartphone app–guided joint self-assessment tool showed modest agreement with physician joint counts, with higher correlation for tender than swollen joints. Patient-reported counts were consistently higher than physician assessments and led to composite disease activity scores that were effective at identifying low disease activity or remission but tended to overestimate active disease, with high positive predictive value (95%) but low negative predictive value (50%).