To develop, validate, and report on the use of a retrospective proxy-reported questionnaire measuring health-related quality of life (HRQoL) in the end-of-life (EOL) phase of high-grade glioma (HGG) patients. Items relevant for the defined construct were selected using existing questionnaires, topics identified as important in literature, and expert opinion (experienced neuro-oncologists and EOL experts). Psychometric properties, content validity and internal consistency, were determined and the questionnaire was subsequently adapted. Proxy-reported HRQoL data of HGG patients in the EOL, including changes over time, were analyzed. Twenty-nine items were selected covering seven domains; physical comfort, physical and cognitive functioning, psychological, social and spiritual well-being, and overall quality of life. Relatives of 83 deceased HGG patients completed the questionnaire. Content validity was assessed to be adequate. Internal consistency in the domains varied from reasonable to good. Two items were excluded due to poor psychometric properties. Symptom burden increased (p < 0.01), except for nausea (p = 0.058), as death approached. Cognitive, physical and psychological functioning deteriorated over time (all p < 0.01). Acceptance of disease seemed to increase slightly towards death, but this was not significant (p = 0.058). Participating in social activities and family life was rated as poor (≤ 50), whereas received support from their social environment and dying with dignity were rated as good (>50). Overall quality of life was rated as poor, mean (SD) of 29 (26). Measuring HRQoL at the EOL of HGG patients with a retrospective, proxy-reported questionnaire was feasible, yielding a validated instrument. HRQoL was reported as poor and deteriorated as death approached.