Introduction Hyperosmolar therapy is crucial for the management of cerebral oedema and high intracranial pressure. Mannitol and hypertonic saline (HTS) at different concentrations are commonly used in clinical practice, but evidence of their use is mostly derived from studies about traumatic brain injury. The aim of this study was to evaluate the efficacy and safety of hyperosmolar therapy for severe aneurysmal subarachnoid haemorrhage (SAH).Methods and design This is a prospective multicentre observational study in which adult patients with severe aneurysmal SAH (Hunt-Hess score >= 3) will be enrolled. At least 124 patients will be enrolled over 2 years. Each enrolled patient will be treated in accordance with relevant management guidelines. The hypertonic agent used will be 20% mannitol (3 mL/kg) or 10% sodium chloride (HTS) (1 mL/kg). The choice and daily dose of the hypertonic agent will be determined by the attending doctor. The primary outcome will be the modified Rankin scale (mRS) score at 30 days after ictus. The secondary outcomes will be the mRS score at 6 months and the incidence of complications during hospitalisation, including phlebitis, acute kidney injury (AKI), hypokalaemia, hyperkalaemia, hyponatraemia, hypernatraemia, vessel spasm, delayed cerebral ischaemia (DCI), subdural effusion and hydrocephalus. An mRS score 0-2 will indicate a favourable outcome, whereas an mRS score 3-6 will indicate an unfavourable outcome. Logistic analysis will be performed to evaluate the association between the hyperosmolar agent and outcome (favourable or unfavourable).Discussions The aim of this multicentre observational study is to provide evidence on the efficacy and safety of hyperosmolar therapy for severe aneurysmal SAH.Ethics and dissemination This study was approved by the Ethics Committee of Xuanwu Hospital (approval date: 10 July 2023) under protocol V.2.0 (version date: 26 April 2023) (reference number: [2023]086). The results will be published in a peer-reviewed journal and reported at relevant professional conferences.Trial registration number NCT05858060.