Sinonasal metastases from prostatic adenocarcinoma are exceptional and may represent a major diagnostic challenge, particularly in the presence of neuroendocrine differentiation. Ethmoidal involvement is especially rare and may mimic a primary sinonasal malignancy clinically, radiologically, and histologically. We report the case of an elderly man presenting with sinonasal symptoms related to a destructive ethmoidal mass. Histopathological examination revealed an infiltrative malignant proliferation composed of irregular glandular structures associated with a focal neuroendocrine component arranged in solid nests and trabeculae. Tumor cells displayed enlarged nuclei with conspicuous nucleoli. Immunohistochemically, tumor cells were negative for CK7 and CK20, while Prostein expression supported a prostatic origin. Synaptophysin and chromogranin were positive within the neuroendocrine component. Subsequent prostate biopsies confirmed the diagnosis of prostatic adenocarcinoma with neuroendocrine differentiation. This case highlights the importance of integrating morphology, immunohistochemistry, and clinicoradiological findings when evaluating unusual sinonasal tumors. Metastatic prostate carcinoma should be considered in destructive sinonasal lesions occurring in elderly men, even in the absence of a known prostatic malignancy.