The objectives of this study were to examine sex and age differences in fundamental motor skills (FMS) and to describe the prevalence of low motor proficiency and mastery competence. The Test of Gross Motor Development—Second Edition was used to assess 2,377 children (3–10 years old) from eight states and 75 schools in Brazil. The results showed that (a) boys are more proficient than girls in the majority of FMS, (b) FMS development begins to plateau at age 7, (c) low motor proficiency is present at age 10 for several FMS, and (d) mastery competence was achieved by only a small number of children. These findings suggest that increased opportunities to engage in physical activity that promotes FMS competence are needed.