Background <p>Globally, many countries are experiencing rapid aging of their working populations. Moreover, older workers are particularly susceptible to poor employment conditions, which can contribute to health inequalities.</p> Objective <p>We explored the association between precarious employment (PE) and cognitive decline.</p> Design <p>A longitudinal analysis of a cohort of Korean workers.</p> Participants <p>We included a nationwide sample of Korean adults aged ≥45 years (<i>n</i> = 2137), with repeated measurements conducted biennially from 2006 to 2020 (6290 observations).</p> Main Measures <p>PE was assessed based on three dimensions: employment insecurity, income inadequacy, and a lack of rights and protection. A summative score was used to categorize an individual’s PE into quartiles (lowest, low, high, and highest). Cognitive decline was defined as a decrease in the Mini-Mental State Examination score by ≥3 after a 2-year follow-up. Generalized estimating equations were used to determine the association between PE and cognitive decline. We estimated odds ratios (ORs) and 95% confidence intervals (CIs).</p> Key Results <p>The mean age was 51.6 years, and the mean baseline MMSE score was 28.7. During the study period, cognitive decline was observed in 15.1% of the observations over a 2-year follow-up. Regarding each PE element, workers with the lowest wage (&lt;60% of the median) was associated with increased odds of experiencing cognitive decline (OR, 1.45; 95% CI, 1.02–2.06). Based on a summative score, workers with high or the highest PE levels had increased odds of experiencing cognitive decline (OR, 1.38; 95% CI, 1.07–1.77 for high PE and OR, 1.51; 95% CI, 1.14–1.99 for the highest PE) compared with workers with the lowest PE level.</p> Conclusions <p>This study suggests that PE is a risk factor for cognitive decline in workers. Enhancing employment conditions for older adults is crucial for promoting healthy aging.</p>