How does social media affect the composition of political protests in the United States? Using early adoption of Twitter as a plausibly exogenous source of variation in county-level Twitter penetration, and comprehensive data on protest events, this paper finds that Twitter penetration increases the frequency of protests overall, but also radicalizes them. Twitter disproportionately fuels protests with participation of “extreme” groups – groups that are particularly militant, radical, or hateful – both on the political left and right. Survey data suggests coordination is not the only mechanism: County-level Twitter penetration also intensifies opinions, and may increase willingness to justify violence.