Background: Secondhand smoke (SHS) is an established risk factor for cardiovascular disease and cancer, but its association with depressive symptoms in adults is unclear, and Korean evidence is rarely reviewed. Methods: Following PRISMA, we searched PubMed, Embase, KMBASE and RISS; 747 records were identified and, after title, abstract and full-text screening, 17 studies were included. PICO: adult (≥19 years) non-smokers; SHS exposure by self-report (home, workplace, public places) or cotinine; non-exposed comparators; depressive symptoms or disorders (PHQ-9, BDI, EPDS). Risk of bias was assessed with ROBINS-E. Results: Studies spanned seven countries including Korea. Most reported a significant association between SHS exposure and depression, with odds ratios generally 1.3-2.2. KNHANES and CLHLS showed dose-response patterns by exposure duration and setting; exposure in all three settings raised risk about threefold (adjusted OR 3.06). Cotinine studies corroborated self-report. Most were cross-sectional; one cohort of 8,842 pregnant women linked passive smoking during pregnancy to postpartum depression (OR 1.43). No study was rated low overall risk of bias (some concerns 9, high 7, very high 1). Conclusion: SHS exposure is consistently associated with depressive symptoms in adults, although risk of bias limits certainty. Standardized measures and longitudinal designs are needed; reducing SHS exposure may support mental as well as physical health.