Most Americans think that the country is politically divided and polarization will only get worse, not better. Such perceptions of polarization are widespread, but we do not know enough about their effects, especially those unrelated to political variables. This study examines the consequences of perceived polarization for levels of social trust in the United States. Trust in fellow citizens is the backbone of a well-functioning democracy, given its role in promoting social cohesion and facilitating collective action. Using nationally representative panel data, as well as an original survey experiment, I find that perceived polarization directly undermines Americans’ trust in each other. A belief that members of society share common values fosters social trust, but perceptions of partisan divisions and polarization make people less trusting of their fellow citizens. Due to perceived polarization, people are less likely to believe that others can be trusted to do the right thing, which in turn decreases their willingness to cooperate for good causes. I discuss the implications of these findings for society’s ability to work together toward common goals.