Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed drugs for mental disorders in critically ill patients. We performed a retrospective cohort study to investigate the association between pre-ICU use of SSRIs and mortality in critically ill adults with mental disorders. We identified critically ill adults with mental disorders based on the Medical Information Mart in Intensive Care-IV database. The exposure was the use of SSRIs during the period after hospital admission and before ICU admission. The outcome was in-hospital mortality. Time-dependent Cox proportional hazards regression models were used to estimate the adjusted hazard ratio (aHR) with 95% confidence interval (CI). To further test the robustness of the results, we performed propensity score matching and marginal structural Cox model estimated by inverse probability of treatment weighting. The original cohort identified 16601 patients. Of those, 2232 (13.4%) received pre-ICU SSRIs, and 14369 (86.6%) did not. Matched cohort obtained 4406 patients, with 2203 patients in each group (SSRIs users vs. non-users). In the original cohort, pre-ICU use of SSRIs was associated with a 24% increase in the hazard for in-hospital mortality (aHR, 1.24; 95% CI, 1.05–1.46;P= 0.010). The results were robust in the matched cohort (aHR, 1.26; 95% CI, 1.02–1.57;P= 0.032) and the weighted cohort (aHR, 1.43; 95% CI, 1.32–1.54;P< 0.001). Pre-ICU use of SSRIs is associated with an increase in the hazard for in-hospital mortality in critically ill adults with mental disorders.