Objective: Considering that epilepsy is common, and knowledge is lacking on its role especially for the prognosis of high-grade gliomas, the objective of this study was to investigate the association between epilepsy prior to glioma diagnosis and survival among glioma patients. Methods: In a nationwide population-based cohort study, we included 3763 adult glioma patients diagnosed between 2009 and 2018 according to the Danish Neuro-Oncology Registry. Information on epilepsy was redeemed through Danish Neuro-Oncology Registry, National Patient Registry, and National Prescription Registry. Cox proportional hazard models with 95% confidence intervals (CIs) were applied to examine hazard ratios (HRs) for the association between epilepsy (< 1 year prior to glioma including epilepsy at onset; 1–10 years prior to glioma; no prior epilepsy) and risk of death, and whether it differed according to tumor grade and size, performance status, and treatment modalities. Results: A 32% decreased risk of death in patients with epilepsy within 1 year prior to glioma compared to no prior epilepsy was found (HR = 0.68; CI 0.63–0.75). A favorable prognosis was seen for epilepsy in all glioma grades: II (HR = 0.55; CI 0.39–0.77), III (HR = 0.59; CI 0.48–0.73), and IV (HR = 0.85; CI 0.77–0.94). Conclusions: Patients with epilepsy within 1 year prior to glioma diagnosis had significant survival benefits compared to patients with no prior epilepsy. This association was significant for both low-grade gliomas (grade II) and high-grade gliomas (grade III and IV). Survival benefits in glioma patients with epilepsy at onset are possibly primarily attributable to tumor-specific histopathology, molecular biomarkers, and early diagnosis.