Llywelyn or ap Llywelyn is a surname, and may refer to:
Angharad ferch Llywelyn (fl.1260), daughter of Llywelyn ab Iorwerth, Prince of Wales
Carmen Llywelyn, American actress and photographer
Dafydd ap Llywelyn (1212–1246), King of Gwynedd
Dafydd ap Llywelyn ap Madog, Welsh poet of the 16th-century
Dafydd Llywelyn (composer) (1939–2013), Welsh pianist and composer
Dafydd Llywelyn (politician) (born 1976), Welsh politician
Desmond Llewelyn (1914–1999), Welsh actor
Elen ferch Llywelyn (c. 1207–1253), daughter of Llywelyn the Great of Gwynedd
Emyr Llywelyn, Welsh political activist
Llywarch ap Llywelyn (fl.1173–1220), Welsh poet
Madog ap Llywelyn (died after 1312), leader of the Welsh revolt of 1294–95
Maredudd ap Llywelyn ap Maredudd ap Cynan, Welsh prince of the 13th century
Morgan Llywelyn (born 1937), American-Irish author
Owain ap Llywelyn ab y Moel (fl.1470–1500), Welsh poet from Powys
Robin Llywelyn (born 1958), Welsh novelist