Seiriol (/ˈsaɪriɒl/, Welsh pronunciation: [ˈsɛirjɔl]) was an early 6th-century saint, who created a cell at Penmon Priory on Anglesey, off the coast of north Wales.

[3] According to legend, Seiriol and Saint Cybi were good friends,[2] and would meet weekly near Llanerchymedd, at the Clorach wells.

[7] The Russian Orthodox Diocese of Sourozh of Great Britain and Ireland (Moscow Patriarchate) holds an annual pilgrimage to the Holy Well of St. Seiriol in Penmon, most recently on July 23, 2022.

The Liverpool and North Wales Steamship Company steamer St Seiriol ran excursions to the Isle of Man.

St Seiriol is depicted in a Venetian Glass mosaic by Gary Drostle at the western end of The Celtic Gateway footbridge in Holyhead.