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 polarization largely did not take place among social media users. Instead, following the arrival of 3G, experienced internet and social media users from both parties became more pro-Democratic, whereas less-experienced users became more proRepublican. This divergence is partly driven by differences in news consumption between the two groups: after the arrival of 3G, experienced internet users decreased their consumption of Fox News, increased their consumption of CNN, and increased their political knowledge. Polarization also increased due to a political realignment of voters: wealthy, well-educated people became more liberal; poor, uneducated people—more conservative.