Background: Effective physical activity interventions are needed for children because health behaviors track into adulthood, and risk factors for diseases begin early in life. No study has determined whether an intervention designed using a Community-Based Participatory Research approach can improve moderate to vigorous physical activity (MVPA) and the related psychosocial constructs in underserved children. This study determined whether improvements in MVPA and related psychosocial constructs (self-efficacy, knowledge, beliefs, attitudes, and skills) occurred following a Community-Based Participatory Research intervention in underserved, rural children. It was then determined if these constructs were mediators of MVPA. Methods: Two fifth-grade classes at a school (n = 19 and n = 20) were randomly assigned to an intervention or comparison group. The intervention group participated in a 4-week intervention designed to improve MVPA (wGT3X-BT accelerometer; ActiGraph, Pensacola, FL) and the related psychosocial constructs (written survey). Groups were assessed prior to and immediately following the intervention. Results: There were no differences at baseline between groups. MVPA (30.0 [4.4] min), knowledge, and skill scores were significantly higher in the intervention group compared with the comparison group at follow-up (P < .05). Knowledge and skills were mediating variables of MVPA. Conclusions: Priority should be placed on research that determines the sustained impact of similar Community-Based Participatory Research interventions.