In this article, I outline the online ‘anti-public sphere’ as an object for analysis, defined as that space of online socio-political interaction where discourse routinely and radically flouts the ethical and rational norms of democratic discourse. This is a formerly offline space made newly visible by digital networked media. It includes discursive spaces and forms such as White supremacist websites, anti-climate science forums, militant ‘men’s rights’ sites, anti-immigration Facebook pages, gay hate memes, misogynist trolling, anti-Semitic websites, alt-right websites and ‘truth’ (conspiracy) websites, to name a few, where discussion flouts norms of public debate, rules of argument and requirements for the rational consideration of evidence for its own ends. Building on earlier work on anti-publics by McKenzie Wark and Bart Cammaerts, and working from examples from several different domains of online anti-public discourse, I argue that despite its size and complexity, it is possible and necessary to theorise this heterogeneous discursive field, not least because while such discourse is often dismissed, the meanings developed in such domains increasingly intermingle with and inform everyday democratic discourse. While we tend to think of extreme and irrational online discourse as aberrant and alien to everyday democratic discourse, analysis suggests that such discourse in fact is a precise reflection of an everyday ‘post-normative’ democratic discourse that has itself become deeply inflected with reactionary and populist themes.