The genus Muscidideicus Becker, 1917 is comprised of only one species, M. praetextatus (Haliday, 1855) reported in west Palearctic region. The adults were typically found on sandy beach within the intertidal zone. In the survey of Korean fauna, authors found three new to science species of this genus on the west sea side of Korean Peninsula and Jeju Island. Descriptions and figures of external features of new to science species and key to species of the world are provided here.