What explains online radicalization and support for ISIS in the West? Over the past few years, thousands of individuals have radicalized by consuming extremist content online, many of whom eventually traveled overseas to join the Islamic State. This study examines whether anti-Muslim hostility might drive pro-ISIS radicalization in Europe. Using new geo-referenced data on the online behavior of thousands of Islamic State supporters in France, the United Kingdom, Germany, and Belgium, I study whether the intensity of anti-Muslim hostility at the local (neighborhood/municipality) level is linked to pro-ISIS radicalization on Twitter. Results show that local-level measures of anti-Muslim animosity correlate significantly and substantively with indicators of online radicalization, including posting tweets sympathizing with ISIS, describing life in ISIS-controlled territories, discussing foreign fighters, and expressing anti-West sentiment. High-frequency data surrounding events that stir anti-Muslim hostility — terrorist attacks and anti-Muslim protests in Europe — show the same pattern.