Affirmative consent is the idea that someone must ask for, and earn, enthusiastic approval before interacting with someone else. For decades, feminist activists and scholars have used affirmative consent to theorize and prevent sexual assault. In this paper, we ask: Can affirmative consent help to theorize online interaction? Drawing from feminist, legal, and HCI literature, we introduce the feminist theory of affirmative consent and use it to analyze social computing systems. We present affirmative consent’s five core concepts: it is voluntary, informed, revertible, specific, and unburdensome. Using these principles, this paper argues that affirmative consent is both an explanatory and generative theoretical framework. First, affirmative consent is a theoretical abstraction for explaining various problematic phenomena in social platforms—including mass online harassment, revenge porn, and problems with content feeds. Finally, we argue that affirmative consent is a generative theoretical foundation from which to imagine new design ideas for consentful socio-technical systems.