How and why does protest shape participants’ attitudes? We study this issue using panel data gathered both before and after Ukraine’s Euromaidan protests. We argue that frames play a key role in this process. We find that protest participation increased alignment of protesters’ policy preferences with the main protest frames by stabilizing existing attitudes that were in alignment and changing attitudes that conflicted with the dominant frame. Attitudes on core protest issues also became more coherent. We find no comparable effects for issues less central to the protests and all effects vary as the framing and protest environment changed. In addition, we examine the mechanisms of attitude change, showing that while protesters also experience significant increases in efficacy, interest, and participation, these changes only partly explain attitudinal changes. These findings speak to both the short and long-term mechanisms by which protest participation can shape public opinion.