Social capital (SC) has been described as the “glue” that facilitates close social relationships among individuals, groups, and communities. However, few studies have empirically tested the structure of this concept. We report two studies that advance and test a model of SC in youth involved in social organizations. A theory-guided multidimensional representation of SC comprising six dimensions–social trust, interpersonal trust, participation processes, team work, political efficacy, and peer solidarity–was tested in a sample of 377 Chilean youth. Participants (51% males, M age = 21 years) were mostly students (90%) drawn from 41 youth organizations that pursued either prosocial or political goals. Findings from confirmatory factor analysis supported a four-factor instead of the original six-factor model. This structure was confirmed in an independent sample of 150 participants of social and political organizations. The four-factor second-order model included interpersonal trust, participation processes, team work, and peer solidarity as first-order factors.