Depression, increasingly recognized as a potential early indicator and risk factor for cognitive decline and dementia. This study investigates how the timing of depression onset—either early or late after a gastrointestinal (GI) disease diagnosis—affects the risk of developing dementia. A particular focus is placed on the heightened risk of depression in the immediate aftermath of a GI disorder diagnosis, examining how this timing relates to subsequent dementia risk. Utilizing a retrospective cohort analysis of 106,245 patients from the Korean National Health Insurance Service-Elderly database over 18 years, this research analyzes dementia and depression incidences based on ICD-10 codes and treatment records, adjusting for demographic variables, health behaviors, and comorbid conditions. Results indicate a significantly higher dementia incidence in patients with early-onset depression (22.38 per 1000 person-years) compared to non-depressed patients (7.84 per 1000 person-years). Late-onset depression patients showed a dementia incidence rate (14.60 per 1000 person-years) that was elevated compared to non-depressed patients but significantly lower than in early-onset depression cases. Additionally, a temporal gradient was observed, where the risk of dementia decreased as the interval between GI disease diagnosis and depression onset increased. Specifically, the hazard ratio for dementia was highest for depression occurring within o.6 years post-GI diagnosis at 2.76 (95% CI: 2.50-3.03), with a gradual decline in risk for depression onset within 1 and 2 years. These findings suggest that the timing of depression following GI disease diagnosis is crucial, with early-onset depression associated with a higher dementia risk.