Online platforms have emerged as a new kind of regulatory object. In this article, we empirically map the emergence of the field of platform regulation in one country: the United Kingdom (UK). We focus on the 18-month period between September 2018 and February 2020 when an upsurge of regulatory activism reflected increasing sensitivity to national sovereignty in the context of Brexit. Through an empirical–legal content analysis of eight official reports issued by the UK government, parliamentary committees, and regulatory agencies, we code the online harms to which regulation is being asked to respond; identify relevant subject domains of law (such as data protection and privacy, competition, education, media and broadcasting, consumer protection, tax law and financial regulation, intellectual property law, security law); and analyze the agencies referred in the reports for their centrality in the regulatory network and their regulatory powers. Drawing on Bourdieu’s notion of “field,” we observe the emergence of regulators with investigatory and enforcement powers that stand in mutually unstable power relations to each other as well as vis-à-vis shifting executive and legislative interventions. Online platforms appear to acquire authority to exercise state powers.