Princess Eilonwy is a fictional character in Lloyd Alexander's The Chronicles of Prydain. When she is being formal, she calls herself "Princess Eilonwy daughter of Angharad daughter of Regat of the Royal House of Llyr" in reference to her mother and grandmother respectively. Her father Geraint was however a commoner. The women in her line are formidable witches, though she prefers to use the term "enchantress". Eilonwy says little about her male ancestors, other than reporting them all as war leaders.

The name Eilonwy, never appears historically before the book, therefore it is believed to be constructed, based on elements borrowed from actual welsh names.

In the Disney animated film The Black Cauldron, which is loosely based on The Chronicles of Prydain, Eilonwy is voiced by Susan Sheridan.

Template:Spoiler

She is first seen in The Book of Three, in Spiral Castle, the stronghold of Achren, the former queen of all Prydain and mentor to Arawn. At first, she seems to be only a rather spoilt, easily-distracted girl, but she rescues Taran and Fflewddur Fflam (the latter by mistake, since Taran thought the only other person in the dungeons would be Gwydion, his companion, and asked Eilonwy to rescue the other person she described to him).

On their way out of the castle, they pass through a tomb, and Eilonwy retrieves a sword. This sword is Dyrnwyn, a magical sword with a blade that bursts into flame when it is drawn. However, Eilonwy will not allow Taran to examine the sword or draw it; despite her incomplete magical training, she is knowledgeable enough to recognize that the sword is highly magical and best left to experts.

After the events in The Book of Three, Eilonwy comes to live at Caer Dallben, home of Dallben the enchanter and Taran. She accompanies Taran on many of his later adventures, and her courage and determination never falter. Eventually Taran becomes High King of Prydain and marries Eilonwy. She must however renounce her magical powers to be allowed to stay in Prydain, since the magical folk are leaving or withdrawing, as in the end of Lord of the Rings.

Physically, Eilonwy is described as blue-eyed, with red-gold hair. She is very determined to have her own way and quarrels frequently with Taran, although the quarrels do not last long. She is never without her "bauble" - a magical ball that she can make give off light, which sometimes shows things not visible under ordinary lights. Other than this, she can do little with magic, since her studies under Achren were interrupted.