We aimed to examine (a) the construct validity and reliability of the International Fitness Scale (IFIS) in children; and (b) the association of both self-reported and measured fitness with cardiometabolic risk. Cross-sectional study in 1145 Spanish children (9-12 years). We measured body fat, waist circumference, blood pressure, lipids, insulin, and fitness level (reported and measured). A validated cardiometabolic syndrome index was used. An age- and sex-matched sample of 245 children originally not included in the study sample fulfilled IFIS twice for reliability purposes. IFIS was able to correctly classify children according to their measured fitness levels (analysis of covariance, adjusted for sex and age was used). Test-retest reliability of IFIS items was also good, i.e., average weighted Kappa = 0.70. Our data also suggest that both measured and reported cardiorespiratory, and speed and agility fitness were associated with cardiometabolic risk factors, such as adiposity and a cardiometabolic syndrome risk score. The associations for muscular fitness (both reported and measured) differed depending on how it was expressed (i.e., absolute vs relative terms). Our findings suggest that self-reported fitness, as assessed by IFIS, is a valid, reliable, and health-related measure, and it can be a good alternative at population level when physical fitness cannot be measured.