Euroswydd is a figure in Welsh mythology, the father of Nisien and Efnysien by Penarddun, daughter of Beli Mawr. In the Second Branch of the Mabinogi Penarddun is the wife of Llyr, by whom her children are Brân, Branwen, and Manawydan. The circumstances of Nisien and Efnysien's conception are not described, but one of the Welsh Triads mentions that Euroswydd had held Llyr captive as one of the Three Exalted Prisoners of the Island of Britain; it is likely the traditions are connected.