Saint Cwyllog (or Cywyllog)[1] was a Christian holy woman who was active in Anglesey, Wales, in the early 6th century.
He was a king in northern Britain who lost his lands and sought safety with his family in Anglesey, where the ruler Maelgwn Gwynedd gave him land in the north-east of the island, the district known as Twrcelyn.
[1] She is said to have founded St Cwyllog's Church, Llangwyllog, towards the middle of Anglesey but within the area granted to her father, in the 6th century.
[5] Her feast day there has been recorded as being 7 January, although this date does not appear in the Welsh calendars of saints.
The original saint of that location would be Gwrddelw, son of Caw, whose feast day is also 7 January.