Voluntary preventive behaviors are essential to slow the spread of the coronavirus disease 2019 (Covid-19), and such behaviors can be promoted by nudge messaging. In this context, this study investigated the effectiveness of nudge-based messages in increasing individuals’ engagement in preventive behaviors. We employed a large-scale randomized controlled trial involving 0.3 million mobile device users from Tokyo; these users were sent nudge-based messages through display advertising. This approach enabled us to track the GPS-based geolocation history of these users through various apps, in July 2020, when the second wave of Covid-19 hit Japan. Specifically, our study is the first attempt to measure the effect of the nudge intervention effects on the spatial movement behavior of people, by using smartphone’s GPS information. The results revealed that the nudge-based messages increased users’ avoidance of closed spaces, crowded spaces, and close contact during the weekends (characterized by heightened leisure activities, and hence spatial movements). The most effective messages emphasized financial loss aversion. The delivery cost of messages was less than $0.1/person, and the people who received the messages reduced outdoor activities by approximately 52 minutes/weekend day. Our follow-up survey suggests that the nudge-based messages cost 2.5–6.5 % of the monetary compensation given for stay-at-home compliance, which achieves the same result. These findings have implications for the development of government marketing strategies and effective nudge-based interventions to overcome the current pandemic.