Background and Purpose— Arterial vasospasm is a well-known delayed complication of aneurysmal subarachnoid hemorrhage (aSAH). However, no validated biomarker exists to help clinicians discriminating patients with aSAH who will develop vasospasm (VSP ⁺ ) and identifying those who then deserve aggressive preventive therapy. We hypothesized that whole-blood miRNAs could be a source of candidate biomarkers for vasospasm. Methods— Using a next-generation sequencing approach, we performed whole-blood miRNA profiling between VSP ⁺ patients with aSAH and patients who did not develop vasospasm (VSP ⁻ ) in a prospective cohort of 32 patients. Profiling was performed on the admission day and 3 days before vasospasm. Results— Four hundred forty-two miRNAs were highly expressed in whole blood of patients with aSAH. Among them, hsa-miR-3177-3p demonstrated significant ( P =5.9×10 ⁻⁵ ; P _{Bonferroni corrected} =0.03) lower levels in VSP ⁻ compared with VSP ⁺ patients. Looking for whole-blood mRNA correlates of hsa-miR-3177-3p, we observed some evidence that the decrease in hsa-miR-3177-3p levels after aSAH was associated with an increase in LDHA mRNA levels in VSP ⁻ ( P <10 ^–3 ) but not in VSP ⁺ ( P =0.66) patients. Conclusions— Whole-blood miRNA levels of hsa-miR-3177-3p could serve as a biomarker for vasospasm. Clinical Trial Registration— URL: https://www.clinicaltrials.gov . Unique identifier: NCT01779713.