Background: Both underweight and overweight are public health concerns in Japan. Several studies examined the association between health literacy (HL) and obesity status in the general population; however, there is limited information on young adults. In addition, the association between HL and underweight status has not been extensively investigated. Aim: To examine the association between HL and underweight/overweight status among young Japanese adults aged 20 to 39 years. Method: This study was based on a cross-sectional survey of population-representative adults. HL was assessed using a questionnaire validated in Japanese adults. Body mass index (BMI) was calculated using self-reported weight and height. Participants were divided into two groups by HL score using the median score (lower vs. higher HL). The association between HL and underweight (BMI <18.5) or overweight (BMI [greater than or equal to] 25.0) was examined using multinomial logistic regression analyses after adjusting for potential confounders. Results: In total, 476 women and 454 men were included in the analyses. Prevalence of underweight and overweight was 20.8% and 10.3% in women and 8.8% and 20.3% in men, respectively. In women, 45.1% of normal weight, 47.5% of underweight, and 30.6% of overweight had higher HL. Among men, 50.3% of normal weight, 35.0% of underweight, and 44.6% of overweight had higher HL. Bivariate analyses showed no statistically significant association between HL level and underweight/overweight status. Even after adjusting for potential confounders, these associations did not change. Discussion and Conclusion: This study suggests that HL scores may not be associated with underweight or overweight status in Japanese adults.