How has mobile internet affected political polarization in the United States? Using Gallup Daily Poll data covering 1,765,114 individuals in 31,499 ZIP codes between 2008 and 2017, I perform a difference-in-differences analysis and an instrumental-variable design to show that, after gaining access to 3G internet, Democratic voters became more liberal in their political views and increased their support for Democratic congressional candidates and policy priorities, while Republican voters shifted in the opposite direction. This increase in political polarization was partly determined by fake news and misinformation consumption among Republican voters and traditional news consumption among Democratic voters.