National Institute of Fisheries Science conducted a total of 39 sighting surveys for marine mammals over 624 sighting days that covered 20,771 nautical miles in Korean waters during 2011–2020. Marine mammals were detected when 83.9% of the marine environment was less than or equal to three on the Beaufort wind scale. Thirteen species of marine mammals were identified, including two species of pinniped, with a majority of cetacean species. Frequently found species were narrow-ridged finless porpoise, followed by common dolphin, minke whale, Dall"s porpoise, and Pacific white-sided dolphin. Narrow-ridged finless porpoises and minke whales were located in and offshore around the Korean peninsula, common and Pacific white-sided dolphins were exclusively found in the East Sea for most seasons. The other marine mammals were spotted in some seas and for limited durations.