The aim of this study was to investigate the inhibitory activity of angiotensin-converting enzyme (ACE) and antihypertensive effects of soybean protein hydrolysate in spontaneously hypertensive rats (SHRs). The inhibitory activities against ACE with an IC50 of the peptidic fraction were 0.82 (casein), 0.73 (soybean protein isolate), and 0.48 mg/mL (soybean acid-precipitated protein), respectively. Peptic hydrolysate containing 1% NaCl was added to the SHRs’feed (5% of each protein hydrolysate). Systolic blood pressure of the soybean protein hydrolysate-supplemented groups were significantly lower than that of the casein hydrolysate-supplemented group in the study period. These data suggested that soybean protein hydrolysate may retard the development of hypertension in SHRs by its ACE inhibitory effect in vivo.