In Hong Kong’s social unrest of 2019, protesters adopted a variety of strategies to express their demands, including demonstrations on the street, political consumerism, and vandalizing stores of retailers perceived to be ideologically opposed to the movement, among others. When the Hong Kong government began to more forcefully suppress and monitor the protest, data became a contentious object as well as a weapon in the repertoire of political struggle. Taking the protesters’ collective initiative of doxing the policemen and their families as an example, this article examines protesters’ engagement with data during the 2019 Hong Kong Anti-ELAB Movement. Specifically, we explore how movement participants adapted doxing practices in response to shifts in the political opportunity structure namely the passage of the National Security Law 2020 (NSL). This article investigates how the discourses, targets, strategies, and sentiments within the doxing changed following the introduction of the NSL. It also discusses the meanings and challenges of data activism and the contentious nature of doxing under different political opportunity structures.