OBJECTIVES: This study explored the associations between social class, job insecurity, and job strain among Korean workers. METHODS: Data on 6143 participants (253 health care workers, 5113 subway workers, and 777 petrochemical refinery workers) from three Korean job-stress studies were used. Job strain and job insecurity were measured with the job content questionnaire. Job strain was defined as a continuous variable according to the demand-to-control ratio and as a binary variable as the highest quartile of this ratio. Social class was defined by indicators of socioeconomic status. The combined effects of job insecurity and socioeconomic status were examined with generalized linear models and logistic regression models. RESULTS: Job insecurity was relatively higher than in other countries (scale mean 5.8). Higher job insecurity was associated with lower social class, and it appeared to partially mediate the effect of socioeconomic status on job strain. Job insecurity and low social class independently elevated job strain. Job strain was the highest among those with a low social class and job insecurity for each socioeconomic indicator. According to the logistic regression models, the odds ratio for high strain was 2.0 (P<0.05) for low job security and low education, 2.4 (P<0.05) for low job security and low income, and 2.4 (P<0.05) for low job security and low occupational class, when compared with the baseline values. CONCLUSIONS: Low social class is associated with higher job strain. Job insecurity is higher among persons in a lower social class, the highest job strain occurring among workers with both factors. Job insecurity appears to intensify the overall effect of social class on job strain.