Background: Within the new digital health landscape, the rise of health apps creates novel prospects for health promotion. The market is saturated with apps that aim to increase physical activity (PA). Despite the wide distribution and popularity of PA apps, there are limited data on their effectiveness, user experience, and safety of personal data. Objective: The purpose of this review and content analysis was to evaluate the quality of the most popular PA apps on the market using healthcare quality indicators. Methods: The top-ranked 400 free and paid apps from iTunes and Google Play stores were screened. Apps were included if the primary behavior targeted was PA; targeted users were adults; the apps had stand-alone functionality. The apps were downloaded on mobile phones and assessed by two reviewers against the following quality assessment criteria: 1) users' data privacy and security, 2) presence of