Our objective was to examine the independent association between abdominal obesity (waist circumference (WC)) and cardiorespiratory fitness (CRF) with metabolic syndrome (MetS) in 2,197 adults (ages 20-49 years) and 3,223 adolescents (ages 12-19 years). Individuals were stratified by CRF and WC using sex- and age-specific MetS criteria for adolescents and adults. Adolescents had a lower prevalence rate of MetS (5.4% vs. 12.8%) and high WC (15.5% vs. 35.7%), but a higher prevalence rate of low CRF (37.6% vs. 15.9%) than adults. As compared to adolescents and adults with low WC, those with a high WC (odds ratio (OR) = 5.5-16.5, P < 0.001) were more likely to have a clustering of MetS factors than those with low WC (OR = 1.2-3.8, P = 0.3 to <0.001), regardless of fitness level. Conversely, the beneficial effects of having moderate/high CRF on MetS were only observed in individuals with low WC, and not high WC. Thus, in conclusion, both high WC and low CRF are associated with increased odds of MetS in adolescents and adults. However, increased abdominal obesity is more strongly associated with MetS in adolescents and adults.