The purpose of this study was to compare the in-water force of young competitive swimmers using tethered swimming and differential pressure sensors. Thirty-one swimmers (16 girls and 15 boys) were randomly assigned to perform two in-water tests. Swimmers completed two maximum bouts of 25 m front crawl with a differential pressure system and a 30 s maximum bout with an attached load cell (tethered-swimming). The peak force (F_PEAK, in N) of dominant and non-dominant upper limbs was retrieved for further analysis. Comparison between methods revealed significant differences in all force variables (p ≤ 0.05) and the biases (mean differences) were large in girls (F_PEAK dominant, 45.89 N;​ F_PEAK non-dominant, 43.79 N) and boys (F_PEAK dominant, 67.26 N;​ F_PEAK non-dominant, 61.78 N). Despite that, simple linear regression models between the two methods showed significant relationships with a moderate effect in all variables for girls, whereas in boys a high and moderate effect was verified for F_PEAK of dominant and non-dominant limbs (respectively). It seems that using pressure sensors and tethered swimming leads to different F_PEAK values in young competitive, where correction factors are needed to compare data between both methods.