Social capital was examined as a mediator of the association between youth development program participation and gains in self-concept in a sample of 86 primarily African American female adolescents in the Cool Girls, program, and 89 comparisons. Two dimensions of social capital (the diversity of girls’ social networks and the number of life domains in which girls were able to access help) were examined. It was hypothesized that participation in Cool Girls would be associated with increases in social capital and that this would mediate the relationship between participation in Cool Girls and increases in self-concept. Social capital was associated with gains in two dimensions of self-concept regardless of program participation: social acceptance and behavioral conduct. Finally, there was a significant indirect effect of participation on social acceptance and behavioral conduct, mediated through social capital. Implications are discussed.