Ustig is a 6th-century Pre-Congregational Saint of Wales.
He was a child of King Caw of Strathclyde,"Caw", A Welsh Classical Dictionary, p. 129 and according to Frederick Holweck, the brother of Aldate, bishop of Gloucester.Holweck, Frederick George. A Biographical Dictionary of the Saints: With a General Introduction on Hagiology United Kingdom, B. Herder Book Company, 1924. p. 1001 He appears as "Iustic" in the tale Culhwch and Olwen.Baring-Gould, Sabine. The Lives of the British Saints, Vol. 4, C. J. Clark, 1913, p. 348
Baring-Gould mentions another Ustig, descended from Cadell Ddyrnllwg, who was a confessor, along with Dyfrig and Eldad, at Cor Garmon at Llancarfan.
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