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 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 is in fact not a historical Welsh name (unlike many others used in the stories). Lloyd Alexander invented it himself, based on elements borrowed from actual Welsh names.

Eilonwy is notable for her use of unusual similes, such as "If you don't listen to what somebody tells you, it's like putting your fingers in your ears and jumping down a well."

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

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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, who later overthrew Achren and sent her into exile. At first, she seems to be only a rather talkative, 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). She is described as being perhaps one or two years younger than Taran, the protagonist of the series, but Taran's own age is never given so we have no definite idea of how old either of the characters are.

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. We later discover that she is the last living descendant of Llyr Half-Speech, the Sea King, which is why Achren abducted the girl as a small child. She is only one capable of reading a tome which requires the light of her bauble, which, only Eilonwy can cause to light. Because her magical tutoring under Queen Achren was incomplete, her ability to perform magic is severely hindered, though she retains and displays certain inborn abilities throughout the series.

Physically, Eilonwy is described as blue-eyed, with red-gold hair (though the film The Black Cauldron depicts her as more of a basic blonde). She has a good heart, but 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. The Castle of Llyr, the third novel in the series, reveals that this is really the Golden Pelydryn, a magical artifact passed through Eilonwy's family from mother to daughter. The events of that book bring Eilonwy back to her ancestral home, Caer Colur, and give her a greater sense of her ancestry.

The fourth book in the series, Taran Wanderer, sheds light on the ultimate fate of Eilonwy's mother, Princess Angharad. Taran makes this discovery while Eilonwy is being fostered at the foreign court of King Rhuddlum and Queen Teleria, learning to be 'a proper princess.' Whether he ever shared the information with Eilonwy is uncertain, though it seems likely that he refrained from doing so in an effort to spare her anguish. By this time Taran realizes that he's fallen in love with Eilonwy, but he is reluctant to act on it because, having been raised as a commoner while Eilonwy is a royal princess, Taran feels that he really doesn't belong in her world. On the other hand, Eilonwy herself seems to have grown rather fond of Taran by this point.

At the end of the fifth book, The High King, 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.

A sixth book was published in the series, The Foundling and Other Tales from Prydain. Among the stories included therein, readers learn about the circumstances which resulted in Princess Angharad running away from home to marry the commoner Geraint. Although Eilonwy inherited her mother's red-gold hair and magical heritage, she inherited her intensely blue eyes from her father.