In this article, we advance the literature on publics in international politics by exploring the nexus between publicness and big tech companies. This nexus finds a significant expression in the increasing impact of big tech companies to mediate disputes over societal problems, deliver social goods and rearticulate public-private relationships. We develop an analytical framework by combining recent scholarship on assemblage theory and publics, allowing us to understand publicness as enacted in practices which revolve around issues and rearticulate relations of authority and legitimacy. To demonstrate the value of the framework, we show how Microsoft is involved in assembling publicness around cybersecurity. Microsoft does so by problematising and countering state-led cybersecurity activities, questioning the state as a protector of its citizens and proposing governance measures to establish the tech sector as authoritative, and legitimate “first responders.” With this rearticulating of public-private relations, we see the emergence of a political subject for whom security is not solely the right of a citizen secured by the state but also a customer service provided as per a service agreement. The study hence offers important insights into the connection between publicness and cybersecurity, state and big tech relations, and the formation of authority and legitimacy in international politics.