Low skeletal muscle mass and its predictors have been studied widely in older adults. There are few data; however, about their association in young Thai adults. The objectives of this study were to identify risk factors of low relative appendicular skeletal muscle mass (RASM) in young adults by sex-specific groups and to determine optimal cut-off points for low RASM for those adults. This was a cross-sectional study. Healthy urban subjects in Khon Kaen, Thailand who were aged 20-39 year were recruited. Baseline characteristics were collected. Body composition estimation was determined using bioelectrical impedance analysis. One-hundred women and 100 men were recruited for this study. The median RASM was 6.13 kg/m2 in women and 8.01 kg/m2 in men. The cut-off points for low RASM were 5.68 kg/m2 in women and 7.22 kg/m2 in men. For women, increased fat mass (FM), and total body water (TBW) were associated with increased RASM with odds ratios (OR) of 1.04 (95% confidence intervals (CI) 1.02, 1.05) and 1.06 (95%CI 1.02, 1.1). Non-sedentary lifestyles were also related to increased RASM (OR 1.23, 95% CI 1.02, 1.57) whereas current smoking were associated with low RASM (OR 0.45, 95% CI0.21, 0.96). For men, RASM showed a positive association only with FM (OR 1.11, 95% CI 1.07, 1.15) and TBW (OR 1.07, 95% CI 1.04, 1.09). In conclusion, FM and TBW were positively associated with RASM in young Thai adults independent of gender. Current smoking and a sedentary lifestyle were predictors of low RASM in women. The results from this study support the standard cutoff values of low RASM among Asians.