Hydrocephalus is a common complication following aneurysmal subarachnoid hemorrhage (aSAH), associated with increased morbidity and mortality. While its immediate negative impact on cognitive function is well-known, the longitudinal effects, especially in lower-grade aSAH patients, remain unclear. This study aimed to assess these effects. Within the prospective, multicenter "MoCA-DCI study" (ClinicalTrials.gov NCT03032471), patients with a GCS of 13-15 < 72 h post-aSAH underwent serial neuropsychological assessments using the Montreal Cognitive Assessment (MoCA) at baseline (< 72 h post-aSAH), around discharge (14-28 days post-aSAH), and at 3-month follow-up. Standardized MoCA scores were compared to evaluate cognitive outcomes, and the likelihood of a clinically meaningful decline (≥ 2 points) was assessed in patients with and without hydrocephalus. We included 112 patients, mean age 53.9 years (SD 13.9), 66.1% female. Forty patients (35.7%) developed acute hydrocephalus and received external ventricular drainage; 10 of these (25%) required a ventriculo-peritoneal shunt. MoCA z-scores were significantly lower in the hydrocephalus group at baseline (-2.84 vs. -1.12, p < 0.001), at discharge (-3.35 vs. 0.53, p < 0.001), and at 3 months (-0.68 vs. 0.07, p = 0.02). Patients with hydrocephalus were more likely to experience a ≥ 2-point decline from baseline at discharge (OR 2.76, 95% CI 1.16-6.53; p = 0.02) but not at the 3-month follow-up (OR 1.22, 95% CI 0.32-4.62; p = 0.77). Acute hydrocephalus has a negative impact on longitudinal neurocognitive function, yet patients demonstrate improvements until 3-month follow-up. The impairment of cognitive function may be partially recovered as cerebrospinal fluid flow is restored or permanently diverted.