PURPOSE: Child marriages and unions can infringe upon adolescent and youth sexual and reproductive health (AYSRH). Interventions increasingly promote strategies to transform social norms or foster the agency of adolescent girls. Recent empirical studies call for further understanding of how social norms and agency interact in ways that influence these practices, especially in contexts where girls' agency is central. METHODS: A secondary cross-case analysis of three qualitative studies (in Brazil, Guatemala, Honduras) was conducted to inform the investigation of how norms and agency may relate in sustaining or mitigating child marriage. RESULTS: Social norms dictating how girls/young women and how men should act indirectly led to child marriages and unions. The data showed that (1) social norms regulated girls' acceptable actions and contributed to their exercise of "oppositional" agency; (2) social norms promoted girls' "accommodating" agency; and (3) girls exercised "transformative" agency to resist harmful social norms. CONCLUSIONS: Research should advance frameworks to conceptualize how social norms interact with agency in nuanced and context-specific ways. Practitioners should encourage equitable decision-making; offer confidential, adolescent-friendly AYSRH services; and address the social norms of parents, men and boys, and community members.