The objective of this longitudinal study was to examine the relationship between screen time (ST) and children's motor proficiency. The amount of time 113 children spent watching television, using a computer, and playing video games as reported by parents at ages 4, 5, and 7 was measured and children's motor skills were evaluated at age 7 with the short version of the Bruininks-Oseretsky Test of Motor Proficiency Second Edition. The results revealed that children who spent more time in front of a screen at age 4 also do so at ages 5 and 7. A negative relationship was observed between ST at ages 4, 5, and 7 and motor proficiency at age 7. Mediation analysis suggested that the negative effect of ST at age 4 on motor competence was mediated by ST at age 7. This result emphasizes the adverse influence of ST stability on motor proficiency.