PURPOSE: Biological age and chronological age (CA) are both used when assessing maturation status in children and adolescents. Muscular development is influenced by puberty and somatic maturity, an indicator of biological age, which can be estimated by the age at peak height velocity (PHV). However, it is unclear whether muscle morphology is influenced more by biological or chronological age in young judo athletes. METHODS: Anthropometric data [weight (BW) and standing and seated height] were collected from thirty-three youth judo athletes [males: n=23, females: n=10; CA: 12.9±2.7y; experience (EXP): 5.5±2.9y] to predict years from PHV and reported as maturity offset (MO: −0.6±2.2y). Muscle morphology [cross-sectional area (CSA), thickness (MT), pennation angle (PA), fascicle length (FL), and echo-intensity (EI)] of the dominant and non-dominant vastus lateralis (VL) was assessed via ultrasonography. The average of both legs was calculated for each morphological measure. Data were examined using stepwise regression to determine if CA, MO, or EXP were predictive of muscle morphology. Partial correlations were used to determine relationships among the identified predictive measures and muscle morphology. RESULTS: MO was shown to be the best predictor (p<0.001) of CSA (R2=0.748, SEE=3.807), and MT (R2=0.535, SEE=0.220), while CA was shown to be the best predictor (p<0.001) of FL (R2=0.602, SEE=0.665). CA was the best predictor of EI (R2=0.287, SEE=14.055, p=0.004), while the inclusion of EXP increased the predictive ability of the model by 23.5% (R2=0.522, SEE=11.741, p<0.001). None of the selected variables were predictive of PA. After accounting for BW, significant relationships were observed between MO and CSA (r=0.394, p=0.047), EI (r=−0.409, p=0.038), and FL (r=0.461, p=0.018); between EXP and EI (r=0.436, p=0.026); and between CA and EI (r=−0.574, p=0.002) and FL (r=0.504, p=0.009). CONCLUSION: Biological age (i.e., maturity offset) appears to be indicative of lower body muscle size, whereas chronological age, and potentially years of experience, appears to be more indicative of lower body muscle composition among youth judo athletes. However, in this cross-sectional sample, ultrasound-derived pennation angle did not appear to be related to age, maturation status, or years of experience.