In the context of rapid urbanization, public open spaces in dense urban areas are critical built environment elements to support active lifestyles. Several reviews have explored the associations of public open space attributes with physically active and sedentary behavior. However, few reviews have included studies from Asia, and no studies have focused on dense urban areas. This systematic review analyzed 18 observational studies investigating associations between public open space attributes with physical activity and sedentary behavior in dense urban areas of East Asian countries, including Japan, Taiwan, China, and Hong Kong. We found that closer distance to and a greater number of public open spaces and features within them were positively associated with leisure-time physical activity. Places near water features and corridors within public open spaces were associated with more sedentary behavior. These findings inform landscape and urban design guidelines for (re)designing public open spaces to support active lifestyles in high dense urban areas.