We study whether social media can reinforce hatred of minorities, with a focus on Donald Trump’s political rise. We show that the increase in anti-Muslim sentiment since the start of Trump’s presidential campaign has been concentrated in counties with high Twitter usage. To establish causality, we develop an identification strategy based on Twitter’s early adopters at the South by Southwest (SXSW) festival, which marked a turning point in the site’s popularity. Instrumenting with the locations of SXSW followers in March 2007, while controlling for the locations of SXSW followers who joined in previous months, we find that a one standard deviation increase in Twitter usage is associated with a 38% larger increase in anti-Muslim hate crimes since Trump’s campaign start. We also show that Trump’s tweets about Islam-related topics are highly correlated with anti-Muslim hate crimes after his presidential campaign, but not before. These correlations persist in an instrumental variable framework exploiting that Trump is more likely to tweet about Muslims on days when he plays golf. Trump’s tweets also predict higher cable news attention to Muslims, particularly on Fox News.