We aimed to test the hypothesis that serum 25-hydroxyvitamin D3 [25(OH)D] concentration is associated with mental health and life stress measures in young adults, and investigate sex and racial disparities in these associations. This study comprised 327 black and white participants. Depression, trait anxiety, perceived stress, and hostility were measured by validated instruments: Beck Depression Inventory (BDI), State-Trait Anxiety Inventory (STAI), Perceived Stress Scale (PSS), and Cook-Medley Hostility Scale (CMHS). Linear regression was used to estimate correlations between serum 25(OH)D concentration and mental health measurements in total population and in subgroups stratified by sex and race. In this sample (28.2 ± 3.1 years, 48% male, 53% black), serum 25(OH)D concentration was negatively related to BDI, STAI, PSS, total CMHS score and the majority of CMHS subscale scores (p-values < 0.05). Stratified by sex, most of these associations remained significant only in women (p-values < 0.05). Stratified by race, higher 25(OH)D concentrations in the whites were significantly related to lower BDI, STAI, PSS, and CMHS-cynicism subscale (p-values < 0.05); 25(OH)D concentrations in the blacks were only inversely associated with CMHS and most CMHS subscales (p-values < 0.05), but not with BDI, STAI and PSS. We present novel findings of consistent inverse relationships between serum 25(OH)D concentration and various measures of mental health and life stress. Long-term interventional studies are warranted to investigate the roles of vitamin D supplementation in prevention and mitigation of depression, anxiety and psychological stress in young adults.