LC/MS-based phytochemical analysis of an EtOH extract of the roots of rice (Oryza sativa; Gramineae), which takes a crucial role in the stable crop population in Asia, resulted in the isolation of a new lignan, oryzativol C (1), as a minor component. The chemical structure of compound 1 was unambiguously confirmed using spectroscopic evidence (including 1D- and 2D-NMR data), HR-ESI-MS, and CD data analysis. Considering the traditional medicinal efficacy of O. sativa and its importance as a food crop, compound 1 was evaluated for effects on breast cancer cell lines (MDA-MB-231) and on glucose-stimulated insulin secretion in an INS-1 pancreatic β-cell line. Compound 1 showed mild cytotoxicity toward the MDA-MB-231. Furthermore, compound 1 stimulated insulin secretion in INS-1 pancreatic β-cells without inducing cytotoxicity. These results indicate that compound 1 is an active ingredient of O. sativa that offers health benefits including inhibition of breast cancer cell proliferation and hyperglycemia control.