In this study, we identified a novel angiotensin-I-converting enzyme (ACE) inhibitory peptide, YQKFPQYLQY (YQK), derived from bovine casein. Casein was hydrolyzed using pepsin and trypsin. The target peptide, YQK, was separated from the hydrolysate by ultrafiltration and Sephadex G-15chromatography. The IC50 value of YQK was 11.1 μM. YQK retained its ACE inhibitory activity under various temperature and pH conditions. It was also stable against the digestive enzymes pepsin and trypsin. The Lineweaver–Burk plot suggested that the inhibitory mode of YQK was competitive. Furthermore, its antihypertensive effects in spontaneously hypertensive rats (SHRs) also revealed that oral administration of YQK can significantly decrease systolic blood pressure. These results suggested that YQK may have potential applications in functional foods or pharmaceuticals as an antihypertensive agent.