The aim of this study has been to evaluate the efficacy of the IL-5 receptor blocker benralizumab on chronic rhinosinusitis with nasal polyposis (CRSwNP), associated with severe eosinophilic allergic asthma. Ten patients with severe eosinophilic allergic asthma and CRSwNP were enrolled. Sino-nasal outcome test (SNOT-22), numerical rating scale (NRS), endoscopic nasal polyp score, Lund Mackey CT (computed tomography) score, and blood eosinophil count were measured at baseline and after 24 weeks of treatment with benralizumab. All the above clinical, endoscopic, imaging, and hematological parameters significantly improved after 24 weeks of treatment with benralizumab. In particular, SNOT-22 decreased from 61.10 ± 17.20 to 26.30 ± 19.74 ( P < 0.001), NRS decreased from 7.20 ± 1.55 to 3.40 ± 2.22 ( P < 0.001), the endoscopic polyp nasal score decreased from 4.20 ± 1.32 to 2.50 ± 1.78 ( P < 0.001), the Lund-Mackay CT score decreased from 16.60 ± 5.50 to 6.90 ± 5.99 ( P < 0.001), and blood eosinophil count decreased from 807.3 ± 271.1 cells/μL to 0 cells/μL ( P < 0.0001). These results strongly suggest that benralizumab exerted a very effective therapeutic action on CRSwNP associated with severe asthma, thus improving nasal symptoms and decreasing polyp size.