Angiotensin-I-converting enzyme inhibitory peptides were isolated from Cassia obtusifolia seeds by alcalase hydrolysis. Using ultrafiltration, the peptides were divided into four fractions (<1, 1–3, 3–5, >5 kDa). The fraction below 1 kDa exhibited the appropriate ACE inhibition (IC50 = 65.88 μg/mL), and was further purified by gel filtration chromatography, which displayed better angiotensin-I-converting enzyme inhibitory activity (IC50 = 53.67 μg/mL). The amino acid sequences of three novel angiotensin-I-converting enzyme inhibitory peptides were identified by liquid chromatography with tandem mass spectrometry as follows: IFPGCAN (IC50 = 23.71 μM), TEDFLTQ (IC50 = 32.82 μM), and GDEGSGGIIR (IC50 = 18.21 μM). Further experiments demonstrated that IFPGCAN and TEDFLTQ were competitive inhibitors, while GDEGSGGIIR was a noncompetitive inhibitor of the angiotensin-I-converting enzyme. Three peptides could form hydrogen bonds with the active site of the angiotensin-I-converting enzyme, according to molecular docking simulations of their interaction with the angiotensin-I-converting enzyme.