Informed by a positive youth development perspective, the current study develops and validates a brief social capital scale for adolescents and emerging adults from racially and ethnically diverse backgrounds. We first detail the development of the scale and examine its factor structure using exploratory factor analysis. Second, using a new and independent sample, we use confirmatory factor analysis to replicate and confirm the factor structure of the scale. Finally, we conduct measurement invariance by gender, age, and race-ethnicity to determine if the scale held similar meaning for participants from different groups. Findings support a multidimensional scale of youth social capital that captures three important elements: network strength, network diversity, and network mobilization. The creation of this scale expands possibilities for researchers and practitioners by providing a comprehensive and actionable measure for empirically assessing youth social capital, which can be leveraged to support positive youth development.