thumb|Ivo portrayed by Rogier van der Weyden
thumb|The relics of Saints Ivo and [[Tudwal|Tugdual in a procession at the gate of Tréguier's cathedral in 2005. In the reliquary is the skull of Saint Ivo]]

Saint Ivo of Kermartin (17 October 1253 – 19 May 1303), also known Yvo or Ives, as Erwann (in Breton) and as Yves Hélory (also Helori or Heloury in French), was a parish priest among the poor of Louannec, the only one of his station to be canonized in the Middle Ages. He is the patron of Brittany, lawyers, and abandoned children. His feast day is May 19. Poetically, he is referred to as "Advocate of the Poor."

Born at Kermartin, a manor near Tréguier in Brittany, Ivo was the son of Helori, lord of Kermartin, and Azo du Kenquis. In 1267 Ivo was sent to the University of Paris, where he graduated in civil law. He went to Orléans in 1277 to study Canon law. On his return to Brittany having received minor orders he was appointed "official", the title given to an ecclesiastical judge, of the archdeanery of Rennes (1280); meanwhile he studied Scripture, and there are strong reasons for believing that he joined the Franciscan Tertiaries sometime later at Guingamp. He was soon invited by the Bishop of Tréguier to become his official, and accepted the offer in 1284. He displayed great zeal and rectitude in the discharge of his duty and did not hesitate to resist the unjust taxation of the king, which he considered an encroachment on the rights of the Church; by his charity he gained the title of advocate and patron of the poor. Having been ordained he was appointed to the parish of Tredrez in 1285 and eight years later to Louannec, where he died of natural causes, after a life of hard work and repeated fasting. He was buried in Tréguier, and on his tomb was supposedly inscribed in Latin: Sanctus Ivo erat Brito / Advocatus et non latro / Res miranda populo. Roughly translated, this means: "Saint Ivo was Breton / A lawyer and not a thief / A marvelous thing to the people." It is a quip about the reputation lawyers have for thievery.

Ivo was canonized in June 1347 by Clement VI at the urging of Philip I, Duke of Burgundy. At the inquest into his sanctity in 1331, many of his parishioners testified as to his goodness, that he preached regularly in both chapel and field, and that under him "the people of the land became twice as good as they had been before". The connexion between religion and good behaviour was especially stressed in his sermons and he is reported to have "chased immorality and sin from the village of Louannec". At the time he was believed to have been a Franciscan tertiary. Shortly after 1362, the future saint Jeanne-Marie de Maillé reported a vision of Yves (and an ecstasy, raptus), during which he told her, "If you are willing to abandon the world, you will taste here on earth the joys of heaven."

Ivo is often presented with a purse in his right hand (for all the money he gave to the poor during his life) and a rolled paper in the other hand (for his charge as a judge). Another popular representation of is Ivo between a rich man and a poor one. The churches of Sant'Ivo alla Sapienza and Sant'Ivo dei Bretoni in Rome are dedicated to him.