Diagnoses of infectious diseases are being transformed as mass self‐testing using rapid antigen tests (RATs) is increasingly integrated into public health. Widely used during the COVID‐19 pandemic, RATs are claimed to have many advantages over 'gold‐standard' polymerase chain reaction tests, especially their ease of use and production of quick results. Yet, while laboratory studies indicate the value of RATs in detecting the SARS‐CoV‐2 virus antigen, uncertainty surrounds their deployment and ultimate effectiveness in stemming infections. This article applies the analytic lens of biological citizenship (or bio‐citizenship) to explore Australia's experience of implementing a RAT‐based mass self‐testing strategy to manage COVID‐19. Drawing on Annemarie Mol's (1999, The Sociological Review, 47(1), 74–89) concept of ontological politics and analysing government statements, scientific articles and news media reporting published during a critical juncture of the strategy's implementation, we explore the kind of bio‐citizenship implied by this strategy. Our analysis suggests the emergence of what we call liminal bio‐citizenship, whereby citizens are made responsible for self‐managing infection risk without the diagnostic certitude this demands. We discuss how the different realities of mass self‐testing interact to reinforce this liminal citizenship and consider the implications for the sociology of diagnosis. [ABSTRACT FROM AUTHOR]