Background: Adult flatfoot leads to injury and decreased quality of life. The most widely applied noninvasive approaches are wearing foot orthoses or exercising. Both interventions raise controversy about reducing pain and neutralizing foot posture. This study investigated the impact of foot orthoses and exercise on pain and navicular drop (present for foot posture). Methods: Four databases were used: MEDLINE, PubMed, Web of Science, and Cochrane, from the earliest records to November 2020. Randomized controlled studies focused on adult flatfoot that evaluated the effect of exercise and foot orthoses on pain and navicular drop were extracted. We used data analysis to estimate the relative effect of heterogeneity using I² and publication bias using funnel plots. Results: Ten studies were identified through to November 2020. Active interventions (AIs) were exercise and exercise combined with foot orthoses; passive interventions (PIs) were foot orthoses and added stretching. Both AIs and PIs decreased pain significantly (SMD −0.94, 95% CI −1.35, −0.54 and SMD −1.4, 95% CI −1.87, −0.92). The AIs reduced pain level better than PIs. Controversially, no treatment was found to affect navicular drop. Conclusion: Both exercise and foot orthoses can reduce pain but not realign foot posture. Exercise alone or combined with foot orthoses showed a better effect on adult flatfoot than only wearing foot orthoses. Active intervention was shown to have better efficacy in reducing pain than passive intervention.