Background and purpose Decompressive hemicraniectomy (DH) reduces mortality of large middle cerebral artery (MCA) territory infarcts. Survivors are at high risk of poststroke seizures (PSSs). This study aims to describe the incidence of PSSs, to identify associated factors, and to assess their impact on long-term outcomes. Methods We included consecutive patients who underwent DH for large MCA infarcts from May 2005 to December 2019 at Lille University Hospital. Patients were followed up at 3 months, 1 year, and 3 years. We analysed (i) the incidence and associated factors of early onset PSSs (EPSSs) with logistic regression models; (ii) the incidence and associated factors of late onset PSSs (LPSSs) in survivors at 7 days with a univariate Cox proportional hazard regression model for interval-censored data; and (iii) the impact of PSSs (EPSSs and LPSSs) on mortality with univariate and multivariate Cox proportional hazard regression models and modified Rankin Scale at 1 and 3 years, with univariate and adjusted multivariate ordinal logistic regression analyses. Results Of 248 patients (150 men, 60.5%; mean age = 50.4 ± 9.6 years), 106 (42.7%) presented PSSs (six inaugural seizures, 22 EPSSs, 78 LPSSs) during follow-up. The PSS cumulative incidence was 12.3% at 7 days, 24.9% at 3 months, 49.8% at 1 years, and 54.8% at 3 years. No predictor was significantly associated with either EPSSs or LPSSs. PSSs did not significantly impact mortality and long-term functional outcome. Conclusions The incidence of PSSs after DH is high, reaching more than 50% 3 years after stroke, but PSSs did not influence long-term mortality or functional outcome.