Online media sources are prominent ways in which health information is spread. The accuracy and credibility of such sources ranges widely, with misleading statements, misreported results of studies, and lack of references causing health misinformation to become a growing problem. Prior research on health-related misinformation related to topics including vaccines, nutrition, and cancer, however, has excluded physical activity despite it being highly searched and discussed online. This systematic review was designed to synthesize the existing literature focused on physical activity misinformation online in accordance with PRISMA guidelines. PubMed, Cochrane, Web of Science (WoS) and Scopus databases were searched for records published between January 2016 and December 2021, which yielded 3,217 articles. After reviewing titles and abstracts, independent reviewers selected on 117 articles for full-text review. Fourteen articles met full inclusion criteria. Of those, eight addressed physical activity misinformation pertaining to specific diseases or conditions, four involved general physical activity messaging and believability, one focused on weight-loss, and one addressed workplace sitting guidelines. The social media platforms YouTube (n=4), Facebook (n=2), and Instagram (n=1) were studied, whereas other articles (n=7) analyzed media that hadn't explicitly been posted to social media but could be shared widely online. Four articles reported research with actively recruited participants and the remaining 10 articles analyzed readily available online content including news articles, websites, magazine articles and advertisements, or social media. Nine articles reported at least one measure of misinformation prevalence, while six reported a metric of reach, and only one study reported a measure of misinformation spread. Only two articles explicitly addressed the spread of physical activity misinformation online. Given the ubiquity of physical activity information online and the high potential for misinformation, we found the number of relevant articles to be lower than expected which warrants further investigation into the extent and effects of physical activity misinformation online.