Argulus coregoni Thorell, 1864 was found in late July 2019 to parasitize the body surface of masu salmon, Oncorhynchus masou masou (Brevoort, 1856), reared for seed production at the Ishikawa Prefecture Fisheries Research Center in Kaga, Ishi- kawa Prefecture, central Japan. This represents the first record of A. coregoni from Ishikawa Prefecture. Sixty and 12 individuals of A. coregoni were found each on two masu salmon of 20.3 and 21.3 cm in standard length. These individuals consisted of 39 males and 33 females, measuring 2.7–8.6 (mean, 5.0) and 2.8–10.3 (5.1) mm long, respectively, and are considered to have hatched during the spring to early summer in 2019 from overwintered eggs. The condition factor of the fish infected with 60 individuals of A. coregoni was lower than that of the other fish.