Purpose <p>Glioblastoma (GBM) is the most aggressive primary brain tumour in adults. One factor which is widely considered to have protective effects for cognition in age-related decline is cognitive reserve. However, little research has been done into the relationship between cognitive reserve and cognitive function in GBM. We investigated whether a multidimensional construct of cognitive reserve predicts cognitive outcomes before and after surgery.</p> Method <p>43 adult patients with GBM (mean age = 62) took the Cognitive Reserve Index Questionnaire and participated in the OCS-BRIDGE cognitive screen at preoperatively, at 72 h and 6–8 weeks postoperatively. Linear regressions and mediation analyses assessed the relationship between cognitive reserve proxies (education, work and leisure) and cognitive performance across domains.</p> Results <p>Higher overall cognitive reserve significantly predicted better preoperative performance in global deficit, executive function, memory and recognition memory. Mediation analysis revealed that education had a direct effect on executive function, independent of lifestyle activities. Postoperatively, most protective effects of cognitive reserve diminished. However, recognition memory at 72&#xa0;h remained significantly associated with cognitive reserve, specifically work and leisure activities, as revealed by the mediation analysis.</p> Conclusion <p>Cognitive reserve protects against tumour-related cognitive deficits, and in executive function, this is primarily driven by education. In contrast, “active” reserve components, such as work and leisure, may support recognition memory during the acute postoperative phase. These findings highlight the distinct clinical utility of multidimensional reserve assessment for preoperative risk stratification and potential prehabilitation.</p>