Background: Flatfoot is characterised by the falling of the medial longitudinal arch, eversion of the hindfoot and abduction of the loaded forefoot. Furthermore, flatfoot leads to a variety of musculoskeletal symptoms in the lower extremity, such as knee or hip pain. The standard conservative treatment for flatfoot deformity is exercise therapy or treatment with foot orthoses. Foot orthoses are prescribed for various foot complaints. However, the evidence for the provision of foot orthoses is inconsistent. The aim of this systematic review is to synthesize the evidence of foot orthoses for adults with flatfoot. Methods: A computerized search was conducted in August 2021, using the databases PubMed, Scopus, Pedro, Cochrane Library, and the Cochrane Central Register of Controlled Trials. Intervention studies of any design investigating the effects of foot orthoses were included, apart from case studies. Two independent reviewers assessed all search results to identify eligible studies and to assess their methodological quality. Results: A total of 110 studies were identified through the database search. 12 studies met the inclusion criteria and were included in the review. These studies investigated prefabricated and custom-made foot orthoses, evaluating stance and plantar pressure during gait. The sample sizes of the identified studies ranged from 8 to 80. In most of the studies, the methodological quality was low and a lack of information was frequently detected. Conclusion: There is a lack of evidence on the effect of foot orthoses for flatfoot in adults. This review illustrates the importance of conducting randomized controlled trials and the comprehensive development of guidelines for the prescription of foot orthoses. Given the weak evidence available, the common prescription of foot orthoses is somewhat surprising.