Saint Gelert, also known as Celer,  Celert or Kellarth, was an early Celtic saint. Several locations in Wales are believed to bear his name. They include Beddgelert ("Gelert's grave") and the surrounding Gelert Valley and Llangeler ("Gelert's church") where there is a church dedicated to him.

Gelert was a hermit in the late 7th century who lived in a cave near what is now known as the Holy Well of St Celer near Llandysul. In the Dark Ages, pilgrims would travel to the well for healing by Gelert. Eventually a chapel dedicated to St Mary (called "Capel Mair") was erected over the well, of which the ruins still remain. 
It is believed that Gelert was at some time a missionary, evangelizing in Llangeler and Beddgelert. According to modern historical belief, he was martyred in Beddgelert, although this is a misconception simply based on the town name's meaning ("Gelert's grave").
It is believed, however, that Gelert was martyred.