Sport-based positive youth development (SBPYD) programs leverage sport as a context in communities to help youth build competencies and developmental assets, reduce risk factors, and transfer skills learned to other social settings. However, many programs do not fully achieve their objectives due to challenges with implementation fidelity. The purpose of this process evaluation was to (a) examine the implementation fidelity of a SBPYD program across multiple program domains, and (b) explore temporal and environmental factors influencing fidelity. The SBPYD program of interest, LiFEsports, is a four-week summer camp designed to enhance the social and athletic competence of youth experiencing social vulnerabilities. The LiFEsports curriculum consists of four sessions per day of sport-specific instruction, healthy lifestyle lessons, and play-based educational activities designed to teach social skills. Program staff (N = 59) completed four valid and reliable self-report tools (aka "Session Logs") to assess their perceived implementation fidelity across four key domains of the program, including social skills coaching, sport coaching, program curriculum, staff practices. Our analyses consisted of a repeated measures ANOVA and multi-level modeling to assess differences across key program domains, changes in perceptions over time, and factors influencing fidelity. Results demonstrated significant differences in fidelity scores, with staff reporting the highest fidelity implementing staff practices and sport coaching, and lower perceptions of fidelity implementing program curriculum and social skills coaching. Multi-level modeling revealed higher perceptions of fidelity related to social skills coaching and program curriculum during the second and third curricular sessions of the day, whereas staff practices had lower fidelity during the fourth session (compared to the first session). Implementation of sport coaching decreased as the week progressed, whereas implementation of program curriculum and staff practices improved. Staff roles (coach vs counselor), perceived staff training efficacy, level of physical contact in sport (minimal vs high), type of session (sport vs education), social skills taught (introduction vs targeted skills), and group size (separate vs combined) significantly predicted implementation fidelity. Evaluation findings have implications for program design and organization, including priorities to enhance staffs' understanding of the program model through quality training and support; the design of simple lessons with focused activities that are replicated throughout the program; helping staff with making cross-curricular connections, planning and problem solving related to group dynamics, accommodating youth developmental differences; and increasing administrative support and accountabilities toward the end of a program week and day. Practitioners, educators, and evaluators can leverage our findings to inform high-quality programs designed to support youth development.