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, this paper shows that, after getting access to 3G internet, Democratic-leaning voters became more liberal in their political views and increased support for Democratic congressional candidates and policy priorities. Republican-leaning voters shifted in the opposite direction. The direction of the effects is largely determined by how individuals use the internet: following the arrival of 3G, active internet users became more pro-Democratic, less active internet users—pro-Republican. Active internet users also experienced an increase in political knowledge. The increase in polarization partly reflects societal inequalities with educated, wealthy individuals becoming more liberal; uneducated, poor individuals—more conservative.