Seizures at the onset (SAO) of aneurysmal subarachnoid hemorrhage (aSAH) occur in up to one of every five cases. To date, there is no consensus on causal background and clinical value of these early bleeding-related seizures. This study aimed to analyze the predictors and the impact of SAO in aSAH.All aSAH patients from the institutional observational cohort (01/2003-06/2016) were retrospectively reviewed. Patients' charts and emergency protocols from first responders were screened for the occurrence of seizures in the first 24 h after aSAH. Patients' baseline characteristics and occurrence of post-hemorrhagic complications were analyzed. Outcome endpoints included in-hospital mortality and poor outcome at 6-month follow-up (modified Rankin Scale > 3).Of 984 patients included in the final analysis, SAO occurred in 93 cases (9.5%) and were independently associated with younger age ( 38.3 ℃ (p = 0.008). There was an association between SAO and early complications (early infarcts [p = 0.004] and primary decompressive craniectomy [p = 0.024]). Only in the subgroup analysis restricted to the younger individuals, SAO independently predicted poor outcome of aSAH (p = 0.002).Onset seizures following aSAH are rare and most likely related to the severity of early brain injury. Particularly, younger individuals are not only at higher risk for SAO, but are also prone to poor outcome in case of aSAH accompanied with SAO.German clinical trial registry (DRKS, unique identifier: DRKS00008749, 06/09/2015).