Background <p>Understanding the effects of working alone on workers’ mental health may help improve employee well-being; however, research on this topic remains limited. This study investigated the association between working alone and depressive mood among employees in the Republic of Korea.</p> Methods <p>We conducted a cross-sectional analysis using data from the seventh Korean Working Conditions Survey (KWCS). The propensity score-matched sample included 837 employees working alone and 4,185 who were not working alone and were matched at a 1:5 ratio. Univariate logistic regression analyses were conducted to identify risk factors for depressive mood. Finally, multivariate logistic regression analyses were performed to assess the association between working alone and depressive mood after adjusting for sociodemographic, psychosocial, health-related, and workplace characteristics.</p> Results <p>Employees working alone had a higher prevalence of depressive mood than those who were not working alone. In the univariate logistic regression analysis, the odds ratio for the association between working alone and depressive mood was 1.83 (95% CI: 1.57–2.13). In the multivariate analysis, this association remained statistically significant (OR = 1.65, 95% CI: 1.10–2.49).</p> Conclusions <p>This study found that working alone was significantly associated with depressive mood among Korean employees. These findings suggest that working alone should be considered an important workplace factor in efforts to promote workers’ mental health.</p>