Hyponatraemia is a common complication after aneurysmal subarachnoid haemorrhage (aSAH) and is associated with worse outcomes. Fludrocortisone, a synthetic mineralocorticoid, may be an effective treatment for hyponatraemia, but its effect on clinical outcomes is unknown. The objective of this study was to describe the study protocol for the Fludrocortisone in Aneurysmal Subarachnoid Haemorrhage (FLASH) trial. The FLASH trial is a phase three randomised, blinded, placebo-controlled, multicentre trial comparing 14 days of treatment with fludrocortisone to matching placebo in adult patients with acute aSAH at hospitals in Australia, New Zealand, and the United Kingdom. The planned sample size is 524 patients. The primary outcome measure is the Modified Rankin Scale score assessed at 6 months after randomisation. The secondary outcome is the Subarachnoid Haemorrhage Outcome Tool score assessed at the same time point. Tertiary outcomes are based on international guidelines for core outcome sets and include economic and quality-of-life analyses. Prespecified subgroups for analysis will comprise aSAH severity and the presence of hyponatraemia at randomisation. The FLASH trial aims to commence recruitment in May 2025. NCT06409364.