Many social movements face fierce resistance in the form of a countermovement. Therefore, when deciding to become politically active, a movement supporter has to consider both her own movement’s activity and that of the opponent. This paper studies the decision of a movement supporter to attend a protest when faced with a counterprotest. We implement two field experiments among supporters of a right- and left-leaning movement ahead of two protest–counterprotest interactions in Germany. Supporters were exposed to low or high official estimates about their own and the opposing group’s turnout. We find that the size of the opposing group has no effect on supporters’ protest intentions. However, as the own protest gets larger, supporters of the right-leaning movement become less while supporters of the left-leaning movement become more willing to protest. We argue that the difference is best explained by stronger social motives on the political left.