In this study, we show that an inflow of immigrants reduces volunteering, a proxy of social capital investment, in receiving communities. Since the 1960s, there has been a large decrease in social capital in the USA as well as a considerable inflow of immigrants. This increased heterogeneity of US cities may have increased the cost of investing in social capital, and thereby, reduced such investment. By using the current population survey September Volunteer Supplement for 2005–2011, we examine the relationship between the proportion of foreign-born people and social capital investment by US-born individuals, proxied by volunteering. Once we correct for immigrants’ self-selection to different destinations using a supply–push instrumental variable, we find that a 1 standard deviation increase in the proportion of foreign-born individuals in a state reduces the probability of US-born individuals volunteering by 0.09–0.15 standard deviations and cuts number of hours volunteered by 0.13–0.21 standard deviations.