How do protest actions impact public support for social movements? Here we test the claim that extreme protest actions — protest behaviors perceived to be harmful to others, highly disruptive, or both — typically reduce support for social movements. Across 6 experiments, including 3 that were preregistered, participants indicated less support for social movements that used more extreme protest actions. This result obtained across a variety of movements (e.g., animal rights, anti-Trump, anti-abortion) and extreme protest actions (e.g., blocking highways, vandalizing property). Further, in 5 of 6 studies, negative reactions to extreme protest actions also led participants to support the movement’s central cause less, and these effects were largely independent of individuals’ prior ideology or views on the issue. In all studies we found effects were driven by diminished social identification with the movement. In Studies 4–6, serial mediation analyses detailed a more in-depth model: observers viewed extreme protest actions to be immoral, reducing observers’ emotional connection to the movement and, in turn, reducing identification with and support for the movement. Taken together with prior research showing that extreme protest actions can be effective for applying pressure to institutions and raising awareness of movements, these findings suggest an activist’s dilemma, in which the same protest actions that may offer certain benefits are also likely to undermine popular support for social movements.