Corporate activism is when a firm takes a public stance on a social or political issue. Although the public increasingly expects corporations to engage in activism, public support varies across individual corporate activist campaigns. While social movement theories would predict that the public will support corporate activism in general, organizational authenticity theory suggests that corporate activism may backfire if firms lack type or moral authenticity. Using semi-structured interviews with diverse stakeholders, a national survey on 525 corporate activist statements, and two pre-registered experiments, we find that the public generally supports corporate activism, but that a firm’s type and moral authenticity substitute each other in shaping both attitudes (support for and intention to join) and behaviors (donation and writing letters of support). We contribute to social movement theory by bringing in organizational theories of authenticity to add nuance to our expectations about public support for corporate activism, and we extend research on organizational authenticity by showing an interaction between type and moral authenticity.