Background: Post-hemorrhagic hydrocephalus (PHH) is a frequent complication of aneurysmal subarachnoid hemorrhage (aSAH), yet the relationship between serum magnesium (Mg) level and PHH remains unclear. To our knowledge, this is the first prospective study to specifically examine the association between admission serum Mg level and PHH in aSAH patients. Methods: In this prospective, multicenter study (October 2019–October 2024), 131 patients with confirmed aSAH were enrolled from four neuro-intensive care units. Patients were stratified by admission serum Mg level as <2.2 mg/dL or ≥2.2 mg/dL. The primary outcome was PHH incidence; secondary outcomes were cerebral vasospasm (CV), delayed cerebral ischemia (DCI), and 30-day modified Rankin Scale (mRS) score. Results: Baseline characteristics were similar between groups. Serum Mg ≥2.2 mg/dL was not significantly associated with reduced vasospasm, DCI, or poor functional outcome. However, serum Mg >2.5 mg/dL correlated with lower PHH incidence in univariate analysis (odds ratio, 0.36; P=0.027) but not in multivariate analysis (P=0.136). Independent predictors of PHH were posterior circulation aneurysm, high Fisher grade, and high Hunt and Hess grade. Poor 30-day mRS was independently associated with high Fisher and Hunt and Hess grades. Conclusions Admission serum Mg level was not independently associated with PHH, although a potential protective trend was noted at higher levels (>2.5 mg/dL). These findings suggest a possible role of Mg in PHH prevention. Further prospective trials are warranted to clarify the therapeutic potential of Mg and to establish optimal monitoring and correction strategies in aSAH management.