The objective of this study was to examine the effectiveness of using ultrasonography for sex determination in young carp. Ultrasound images were obtained from anesthetized captive carp aged 11 to 13 months. After sacrifice, the body length and weight, gonadal sex and weight, and gonad-somatic index were measured. Well-developed testes and ovaries filled with granulated eggs were easily distinguishable on ultrasound imaging, based on their visible characteristic morphological features. In contrast, immature, small-sized testes and ovaries lacking granulated eggs were difficult to distinguish. The mean longitudinal thickness of gonads was technically the easiest to scan for measuring gonadal size, and it correlated well with gonadal weight ( r  = 0.91) and gonad-somatic index ( r  = 0.89). Based on the area under the curve of the receiver operating characteristic curve, we determined that 6.2 mm was the cutoff value for mean longitudinal gonadal thickness at which sex could be positively determined by ultrasonography, with a sensitivity of 82.6% and specificity of 100%. Ultrasonography therefore represents an effective and non-invasive diagnostic tool for determining sex and maturity in Japanese ornamental carp.