Background: Physical activity has long been considered an important component of a healthy lifestyle. Although many efforts have been made to promote physical activity, there has yet to be an effective global intervention in physical activity promotion. Some researchers suggest that Pokémon GO, a location-based augmented reality (AR) game, effectively promotes physical activity on a global scale, but the details of the impact are far from clear. Objective: To study the impact of Pokémon GO on players' physical activity, and how the impact varies across players with different physical activity levels. Method: We conducted a field study to investigate Pokémon GO's impacts on physical activity in five types of built environment in Hong Kong. Pokémon GO Players were asked to report their demographics through a survey, and their Pokémon GO behaviours and data of daily walking and running distance were collected from their mobile phones. Participants (n=210) were residents of Hong Kong, aged 13 to 65 years old and playing Pokémon GO using iPhone 5 or 6 series in five selected types of built environment. We measured the average daily walking and running distance over a period of 35 days, from 14 days before to 21 days after installation of Pokémon GO. Multilevel modelling was used to identify and examine the predictors -including Pokémon GO behaviours, weather; demographics and built environment -of Pokémon GO's impact on daily walking and running distance. Results: Average daily walking and running distance increased 18.1% (0.96 km) in the 21 days after participants installed Pokémon GO compared with the distance over the 14 days before installation. However, this impact attenuated over time and is estimated to disappear after 26 days (P<0.001). Multilevel models indicate that Pokémon GO has a stronger and more lasting impact on less physically active players than more physically active ones (P<0.001). Playing Pokémon GO in green space has a significant positive relationship with walking and 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 running distance (P<0.001). Moreover, results show that Pokémon GO behaviours -play Pokémon GO or not, as well as days played, weather (total rainfall, bright sunshine, mean air temperature, and mean wind speed), and demographics (age, gender, income, education, and BMI) -were associated with walking and running distance. Conclusions: Pokémon GO can efficiently activate players, especially less physically active ones, to do physical activity. Pokémon GO has the potential to build new links between humans and green space to encourage people to do physical activity. The results indicate that location-based AR games, such as Pokémon GO, have a great potential to be an effective public health intervention at the global scale.