The main purpose of the study was to explore sex and age specific correlations between self-reported and estimated physical fitness. In this cross-sectional study, we recruited 1 036 secondary-school students (55.3% girls). Self-reported physical fitness was assessed on a 10-point scale, with a higher score indicating better physical fitness. We measured waist circumference, sit-ups in 1 minute, standing long jump and sit-and-reach test and calculated z-scores to obtain objective physical fitness index. Boys performed better in sit-ups in 1 minute and standing long jump tests and had higher waist circumference values. Girls performed better in sit-and-reach test. Overall, boys had higher physical fitness z-score values, compared to girls. Younger boys had better physical fitness perception (r1st grade = 0.61, p<0.001), compared to older ones (r = 0.40-0.50, p<0.001). In girls, correlation coefficient was the highest in the 3rd grade (r = 0.46, p<0.001), followed by the 2ndgrade (r = 0.43, p<0.001), the 1stgrade (r = 0.41, p<0.001) and the 4th grade (r = 0.37, p<0.001). Our study shows moderate sex and age specific correlation between self-reported and estimated physical fitness in a large sample of adolescents.