Celtchar (Celtchair, Celtar, Keltchar is a leading figure in the Ulster Cycle of Irish Mythology. An Ulster warrior, he is described as tall, grey and ugly. He wields a spear, the Lúin Cheltchair, whose lust for blood is so great it has to be dipped in a cauldron of poison to control it.

He was emasculated in an encounter with the Connacht warrior Cet mac Mágach.

He killed Blaí Briugu after the latter slept with his wife, Brig Bretach. As compensation for the murder he had to rid Ulster of three menaces.

The first was Conganchnes, who wanted to avenge the death of his nephew Cú Roí. He had skin like horn, which no weapon could pierce. Celtchar offered him his daughter Níab, who discovered that he could only be killed by hammering red hot spits into the soles of his feet. She passed the information to her father, who did the deed.

The second was a ferocious dog called Luch Donn ("brown mouse"). It had been found as a pup by a widow, who raised it until it was enormous and uncontrollable. It killed all the widow's sheep and cattle, then her sons, and finally the widow herself, and now it would devastate a settlement a night. Celtchar found an alder log, hollowed it out so his arm would fit through it, and boiled it in honey, grease and herbs until it was tough and supple. He approached the dog with the log over his arm, and when the dog bit into it its teeth stuck, enabling Celtchar to kill it.

The third was Dóelchú, Celtchar's own dog. It was found as a pup inside Conganchnes's burial mound, and would only let Celtchar handle it. One day it escaped, and became a menace to the cattle and sheep of Ulster. Celtchar found the dog and called to it, and it came and licked his feet. Reluctantly, Celtchar despatched it with his spear. As he lifted the spear, a drop of the dog's poisonous blood ran down it and through Celtchar's body, killing him.

An earthwork in Downpatrick called Rathkeltair (Irish Ráth Celtchair or Celtchar's Fort), also known as the Mound of Down, preserves Celtchar's name.