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, their support varies across individual corporate activist campaigns. While social movement theories predict that the public will support corporate activism in general, organizational authenticity theory suggests that corporate activism may backfire if a firm lacks type or moral authenticity. Using semi-structured interviews with a variety of different stakeholders, a national survey on 525 corporate Black Lives Matter statements, and two pre-registered experiments, we find that the public generally supports corporate activism, but that a firm’s type and moral authenticity can substitute for one another in shaping the public’s 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 help us explain when and why corporate activism sometimes fails to mobilize support, and we extend research on organizational authenticity by showing an interaction between type and moral authenticity.