Both low socioeconomic status (SES) and diabetes mellitus (DM) are important risk factors for mortality. However, little is known about their combined effects and relative contribution to the mortality risk.From a nationwide cohort provided by the National Health Insurance Service in Korea, 153,075 subjects who were over 30 years of age from 2003 to 2004 were followed-up until 2010. The SESs of the subjects in the DM and non-DM (NDM) groups were categorized into 3 groups (highest 30% as S1, middle 40% as S2, and lowest 30% as S3) based on the subjects' income levels.During the 7.9-year follow-up, 3933 deaths occurred. When the subjects were stratified into 6 groups by their socioeconomic and diabetes status, a linearly increasing pattern of the hazard ratio (HR) of mortality from the higher SES without diabetes group (NDM-S1, as a reference) to the lower SES with diabetes group (DM-S3; HR, 2.04, 95% confidence interval (CI), 1.80-2.36) was observed (P for trend < 0.001). Notably, subjects with DM in the highest SES group (DM-S1) had a significantly higher mortality risk than did non-DM subjects in the lowest SES group (NDM-S3). This pattern was maintained in cause-specific mortality but was more prominent in cardiovascular disease (CVD) and less prominent in cancer mortality. The association was not affected by gender; however, in individuals <60 years of age, the combined effects of SES and DM on mortality were more prominent (DM-S3; HR, 3.68, 95% CI, 2.95-4.60) than in those ≥60 years of age.Low SES and DM were major determinants of mortality and synergistically increased the risks of all-cause, CVD, and cancer mortality.