Sport is a critical context for youth development (Riley & Anderson-Butcher, 2012; Weiss, 2008). Through engagement in sport-based positive youth development (PYD), vulnerable youth can experience relief from risk prevalent in their environment and develop assets to overcome adverse circumstances (Anderson-Butcher, Iachini, Riley, Wade-Mdivanian, Davis & Amorose, 2013; Petitpas, Cornelius, Van Raalte, & Jones, 2005). Organizational effectiveness of sport-based PYD programs is imperative due to the impact programs can have on youths’ successful transition to adulthood.A sport-based PYD program is considered effective when exposure to an intentionally designed intervention, with theoretical justification, produces the desired program outcomes (Dusenbury, Brannigan, Falco, & Hansen, 2003; Law & Shek, 2011; Nation, Crusto, Wandersman, Kumpfer, Seybolt, Morrissey-Kane, & Davino, 2003). Research examining organizational effectiveness, within the context of sport-based PYD, can inform program planning, improve program practices, and enhance PYD. The purpose of the current study was to examine program dosage as a programmatic mechanism of PYD, to determine how program implementation and engagement influences youth development in the context of sport-based PYD.To investigate program dosage as a programmatic mechanism of PYD, within the context of sport-based PYD, data were collected from 52 staff and 417 youth participants engaged in a sport-based PYD summer program. Program staff completed self-assessments of their implementation of the program, including adherence to program structure, use of curriculum, and creation of a caring, mastery-oriented climate. Additionally, youth reported perceptions of program engagement (i.e., behavioral engagement, belonging, leader support), as well as social and athletic competencies (i.e., PYD indicators). Youth attendance was collected as secondary data, as a measure of quantity of program dosage.Confirmatory factor analyses were conducted to test the measurement models, for which implementation of the program curriculum and youth attendance were found to be non-significant measures of quantity of program dosage. To test the relationships between program dosage (i.e., program implementation and engagement) and PYD, structural equation modeling (SEM) was employed. Results found program engagement to be a significant positive mediator of social and athletic competence development. Comparatively, program implementation was found to be a significant positive antecedent of program engagement in the athletic competence model and a non-significant positive antecedent of program engagement in the social competence model. Overall, the study demonstrated PYD in four of the five indicators, including general social competence, effort, teamwork, and athletic competence.Collectively, the program implementation and engagement dimensions of program dosage were found to be significant programmatic mechanisms of PYD. The findings have implications for sport-based PYD programs and practitioners, specifically in the areas of program design and evaluation, as well as staff recruitment, training, and support. Future research should seek to build from the current study to further explore how programs can effectively develop youth through sport.