The balance ability in children with attention deficit hyperactivity disorder-combined type (ADHD-C) has not been fully examined, particularly dynamic sitting balance. Moreover, the findings of some published studies are contradictory. We examined the static and dynamic sitting balance ability in 20 children with ADHD-C (mean age: 9 years 3 months; 18 boys, 2 girls) and 20 age-, sex-, height-, weight-, and IQ-matched healthy and typically developing controls (mean age: 9 years 2 months; 18 boys, 2 girls). The balance subtests of the Movement Assessment Battery for Children (MABC) and the Bruininks-Oseretsky Test of Motor Proficiency (BOTMP) were used to compare the two groups, and a mechanical horseback riding test was recorded using a motion-capture system. Compared with the controls, children with ADHD-C had less-consistent patterns of movement, more deviation of movement area, and less-effective balance strategies during mechanical horseback riding. In addition, their performance on the balance subtests of the MABC and BOTMP were not as well as those of the controls. Our findings suggest that balance ability skill levels in children with ADHD-C were generally not as high as those of the controls in various aspects, including static and dynamic balance.