Previous studies have documented that individual differences in fine and gross motor skills are associated with executive function (EF) skills. This study used an experimental design to test whether participating in cognitively challenging motor skills activities was causally related to improvements in motor skills and two key indicators of school readiness: executive function and early numeracy skills. The motor skill program involved fine and gross motor game-like activities that were delivered in a small group format. Activities were socially engaging and progressively challenged children based on their motor competencies. Fifty-three preschool-aged children participated in 16 motor skill sessions across 8 weeks. There were significant treatment effects for all outcomes, such that children in the treatment condition exhibited significant improvements in motor, EF, and early numeracy skills, compared to their peers in the waitlist control condition. Treatment effects on EF skills were stronger for inhibitory control than working memory. Improvements in numeracy were most pronounced for children with initially lower levels of ability. Motor skill-based interventions are an ecologically valid and developmentally appropriate approach for fostering school readiness skills in early childhood. [For the corresponding grantee submission, see ED611702.]