Objectives: Evidence supports that girls are less proficient than boys at performing ball skills. This study examined the immediate and long-term effects of a ball skill intervention on preschool-age girls' ball skill performance. Design: Randomized controlled trial. Methods: Girls (Mage = 47.24 ± 7.38 months) were randomly assigned to a high autonomy, mastery-based 9-week motor skill intervention (the Children's Health Activity Motor Program; CHAMP, 540 min; n = 38) or a control group (free-play; n = 16). Ball skill proficiency was assessed at pretest, posttest, and retention test(after 9 weeks)using the object control subscale ofthe Test of GrossMotor Development - 2ndEdition. Treatment efficacy was examined using linear mixed models. Two models were fit: one for short-term changes (pretest to posttest) and one for long-term changes (pretest to retention). Results: Linear mixed models revealed a significantly time*treatment interaction for both models. Post hoc analysis confirmed that girls in CHAMP experienced significant gains in ball skills from pretest to posttest (p < .001) and pretest to retention (p < .001). Moreover, girls in CHAMP were no different from the control group at pretest(p > .05) but had significantly higher ball skills scores at both posttest(p < .001) and retention (p < .001). Conclusions: This study demonstrates the positive effects of a ball skill intervention (i.e., CHAMP) on improving girls' ball skills both short- and long-term. Findings suggest that early childhood interventions that focus on the development of ball skills in young girls might be an avenue to improve girls' ball skill performance.