Previous evidence has indicated that resection for recurrent glioblastoma might benefit the prognosis of these patients in terms of overall survival. However, the demonstrated safety profile of this approach is contradictory in the literature and the specific benefits in distinct clinical and molecular patient subgroups remains ill-defined. The aim of this study, therefore, is to compare the effects of resection and best oncological treatment for recurrent glioblastoma as a whole and in clinically important subgroups. This study is an international, multicenter, prospective observational cohort study. Recurrent glioblastoma patients will undergo tumor resection or best oncological treatment at a 1:1 ratio as decided by the tumor board. Primary endpoints are: 1) proportion of patients with NIHSS (National Institute of Health Stroke Scale) deterioration at 6 weeks after surgery and 2) overall survival. Secondary endpoints are: 1) progression-free survival (PFS), 2) NIHSS deterioration at 3 months and 6 months after surgery, 3) health-related quality of life (HRQoL) at 6 weeks, 3 months, and 6 months after surgery, and 4) frequency and severity of Serious Adverse Events (SAEs) in each arm. Estimated total duration of the study is 5 years. Patient inclusion is 4 years, follow-up is 1 year. The study has been approved by the Medical Ethics Committee (METC Zuid-West Holland/Erasmus Medical Center; MEC-2020-0812). The results will be published in peer-reviewed academic journals and disseminated to patient organisations and media.