Background: Maternal overweight/obesity during pregnancy increases offspring's risks of obesity and cardiovascular disease (CVD). A possible pathway is by reduced physical fitness and physical activity (PA) levels in children of overweight/obese mother. We assessed whether maternal prepregnancy overweight/obesity independently determines cardiorespiratory fitness (CRF), muscular strength, moderate-to-vigorous physical activity (MVPA), and sedentary behavior (SB) in 8- to 9-year-old children. We also assessed whether child's fat mass (FM) mediates these associations. Methods: One hundred ninety-four children of Dutch ethnicity aged 8.6 (± 0.4) years were randomly selected from a prospective birth cohort, the Amsterdam Born Children and their Development (ABCD) study. CRF was assessed by the 20-m multistage shuttle run test (20-m MSRT), muscular strength by hand dynamometry, and MVPA and SB by accelerometry. The association of prepregnancy body mass index (BMI) ≥ 25 kg/m2 with these outcome measures was assessed by multivariable linear regressions. Results: Mean (± standard deviation) attained 20-m MSRT stage was 5.3 (± 1.7). Compared with children from normal weight women, children of women with prepregnancy overweight/obesity attained a 0.80 (95% confidence interval: 0.15-1.50) lower stage, adjusted for child's sex and MVPA. This association was not mediated by birthweight or child's FM at age 5 years. Maternal prepregnancy overweight/obesity was not associated with child's muscular strength, MVPA, or SB. Conclusions: Maternal prepregnancy overweight/obesity was associated with reduced childhood CRF, but not with muscular strength, PA, or SB. Birthweight and FM at age 5 years did not mediate this association. Reduced CRF may partly explain the increased CVD risk in children of overweight/obese women.