<p>People’s social networks are fragmenting and partisan animosity is rising. Three studies highlight the connection between social network density and partisan animosity. Americans with denser networks (where more social contacts know each other) appear to often harbor less animosity toward political opponents. One explanation for this association is feelings of unity: dense social networks feel more unified, and people project these feelings of connection and common identity onto distant fellow Americans. Study 1 supports this idea with a representative sample of Americans: social network density predicts feelings of local unity, which predicts feelings of national unity, which predicts warmth toward and willingness to interact with political opponents. Study 2 attempts to manipulate perceived social network density and finds a “causal cascade” of indirect effects culminating in lower partisan animosity, suggesting that more involved interventions may reduce partisan animosity. Study 3 uses nationwide county-level data from the Social Capital Atlas and Project Implicit to reveal that communities with higher social network density exhibit lower partisan animosity. The results suggest that considering the overarching structures of social networks is crucial to understanding partisan animosity, and that fostering social cohesion could help mend political divides.</p>