Adopting plant-based, or vegan, diets can have a number of benefits, including mitigating climate change, promoting animal welfare, or improving public health. In the current research, we use social psychological theory to better understand what motivates vegans to engage in collective action on behalf of this social group - that is, what motivates individuals to promote, or encourage others to adopt, a vegan lifestyle. We develop and test a Social Identity Model of Vegan Activism, which highlights the roles of individuals' social identities, sense of efficacy, emotions and moral convictions in fostering collective action. In two pre-registered studies, the first with self-identified vegans from Australia and the UK (N = 351), and the second with self-identified vegans from the UK and the US (N = 340), we found that individuals more frequently engaged in vegan activism (i.e., actions to promote vegan lifestyles) when they had stronger moral convictions (i.e., deontological or consequentialist), greater collective efficacy (i.e., beliefs that vegans can make a positive difference), anger (i.e., when thinking about the reasons why they are vegan), and identification (both with vegans, and with animals). Deontological and consequentialist moral convictions had significant indirect effects on vegan activism via different mediators. We conclude by discussing the implications and importance of studying dietary behavior from a social identity perspective, including its ability to help explain how and why individuals become motivated to not only adopt a certain (e.g., vegan) lifestyle themselves, but to also ‘act collectively’ on behalf of that shared group membership (e.g., promote vegan-friendly behaviors). We also highlight some key insights for policy makers and campaigners aiming to promote plant-based diets.