We study how rising market integration shaped cooperative culture and behavior in the United States between 1850 and 1920. This period marked a major episode of domestic market integration, when rapid railroad expansion and mass migration created an unprecedented, highly integrated national economy. We combine county-level market access with full-count censuses, newspapers, election returns, tax records, and patent data to construct measures of universalism, tolerance, and generalized trust—cultural traits that support cooperation with strangers—as well as indicators of cooperative behavior and the prevalence of impersonal, mutually beneficial interactions. We find that increased market access fostered impersonal cooperative cultural traits, increased impersonal cooperation, and reduced kin-based support. Using linked census records to track domestic migrants, we find no systematic evidence of selection: counties gaining market access did not attract migrants with stronger premigration universalism or related characteristics. Instead, migrants who moved to these counties adapted rapidly, becoming more universalistic and increasingly engaged in impersonal interactions. Adaptation was concentrated among individuals working in commerce-intensive industries and was associated with improved economic outcomes. Our analysis of mechanisms suggests that exposure to impersonal, mutually beneficial exchange was central to how market integration reshaped cooperative culture and broadened the scope of cooperation. These findings help reconcile competing views on markets’ social consequences: market integration fostered impersonal cooperation while eroding kin-based support.