Only 24% of US youth meet physical activity recommendations set by the Centers for Disease Control and Prevention. Research demonstrates that community-based programs provide underresourced minority youth with opportunities for routine physical activity, although limited work draws from accelerometry data. This study objectively assessed youth physical activity attributable to participation (vs nonparticipation) days in a park-based afterschool program in Miami-Dade County, Miami, FL. Participants' (n = 66; 60% male; 57% white Hispanic, 25% non-Hispanic black, 14% Black Hispanic, mean age = 10.2 y) physical activity was assessed April to May 2019 over 10 days across 7 park sites using Fitbit (Charge 2) devices. Separate repeated-measures multilevel models were developed to assess the relationship between program daily attendance and total (1) moderate to vigorous physical activity minutes and (2) step counts per day. Models adjusted for individual-level age, sex, race/ethnicity, poverty, and clustering by park showed significantly higher moderate to vigorous physical activity minutes (β = 25.33 more minutes per day; 95% confidence interval, 7.0 to 43.7, P < .01) and step counts (β = 4067.8 more steps per day; 95% confidence interval, 3171.8 to 4963.8, P < .001) on days when youth did versus did not attend the program. Study findings suggest that park-based programs may support underserved youth in achieving daily physical activity recommendations.