As part of our study of the isolation of antihypertensive agents derived from natural marine products, the bioactivity of 10 edible Korean seaweeds were screened by angiotensin converting enzyme (ACE) inhibitory and peroxynitrite assays. Among the crude extracts of selected seaweeds, including five Phaeophyta (Ecklonia stolonifera, E. cava, Pelvetia siliquosa, Hizikia fusiforme, and Undaria pinnatifida), four Rhodophyta (Gigartina tenella, Gelidium amansii, Chondria crassicaulis, and Porphyra tenera) and one Chlorophyta (Capsosiphon fulvescens), the ethanol extracts of E. stolonifera, E. cava, P. siliquosa, U. pinnatifida, and G. tenella exhibited significant inhibitory properties against ACE at more than 50% inhibition at a concentration of 163.93 µg/mL. Phloroglucinol 1, eckstolonol 2, eckol 3, phlorofucofuroeckol A 4, and dieckol 5 had been isolated previously, and triphlorethol-A 6 and fucosterol 7 were isolated for the first time from E. stolonifera. Also, the ACE inhibitory and peroxynitrite scavenging properties of phlorotannins 1–6 were evaluated, along with fucosterol 7 obtained from E. stolonifera. Among profound peroxynitrite scavenging compounds 1–6, phlorotannins 3, 4 and 5 were also determined to manifest marked inhibitory activity against ACE, with 50% inhibition concentration (IC50) values of 70.82 ± 0.25, 12.74 ± 0.15, and 34.25 ± 3.56 µM, respectively.