Vitamin D plays pivotal role in bone mineral homeostasis. But the association of vitamin D with muscle mass remains obscure, especially among young adults. Therefore, we assessed the association between muscle mass and 25-hydroxyvitamin D (25[OH]D) in South Korean adults using data from the 2009–2010 Korean National Health and Nutrition Examination Survey (KNHANES). This study involved 12,324 (5,375 males and 6,949 females) participants in the 2009–2010 KNHANES aged 20 y or older. Appendicular skeletal muscle mass (ASM) was measured by dual X-ray absorptiometry. Low muscle mass was defined as an ASM divided by body mass index (BMI) (ASM [kg]÷BMI [kg/m²]) value of <0.789 in males and <0.512 in females. The vitamin D status was evaluated by assaying the serum 25(OH)D level. After adjustment for covariates, low muscle mass was significantly associated with lower 25(OH)D level (odds ratio [OR], 0.55; 95% confidence interval [CI], 0.40–0.75 for 10.0–19.9 ng/mL vs. <10.0 ng/mL; OR, 0.47; 95% CI, 0.33–0.68 for 20.0–29.9 ng/mL vs. <10.0 ng/mL; and OR, 0.39; 95% CI, 0.24–0.64 for ≥30.0 ng/mL vs. <10.0 ng/mL). Moreover, low muscle mass was significantly associated with lower 25(OH)D level in all age groups. In conclusion, low muscle mass was significantly associated with lower 25(OH)D level in South Korean adults in all age groups.