The Owl Service is a low fantasy novel for young adults by Alan Garner, published by Collins in 1967. Set in modern Wales, it is an adaptation of the story of the mythical Welsh woman Blodeuwedd, an "expression of the myth" in the author's words.

Garner won the annual Carnegie Medal from the Library Association, recognising the year's best children's book by a British author. It was named one of the top ten Medal-winning works for the 70th anniversary celebration in 2007, selected by a panel to compose the ballot for a public election of the all-time favourite. Garner also won the second annual Guardian Children's Fiction Prize, a similar award that authors may not win twice. Only six books have won both awards in the 45 years from 1966 to 2011.

The mythical Blodeuwedd is featured in the Fourth Branch of the Mabinogi. She is a woman created from flowers by the king of Gwynedd, Math, and the magician and trickster Gwydion, for a man who was cursed to take no human wife. She betrays her husband Lleu in favour of another man, Gronw, and is turned into an owl as punishment for inducing Gronw to kill Lleu. In Garner's tale three teenagers find themselves re-enacting the story. They awaken the legend by finding plates from a dinner service with an owl pattern, which gives the novel its title.

Henry Z. Walck published the first US edition in 1968.

Roger and Alison are stepbrother and sister. Alison's father died and her mother Margaret has married Clive, a businessman and former RAF officer. Clive's former wife was notoriously unfaithful, bringing shame to the family and a particular kind of pain to his son, Roger. To bond the new family together they are spending a few weeks of the summer in an isolated valley in Wales, a few hours' drive from Aberystwyth. They occupy a fine house formerly owned by Alison's father, subsequently transferred to her to avoid death duty. He in turn inherited it from a cousin, Bertram, who died there in mysterious circumstances around the time Alison was born.

With the house comes Huw Halfbacon, also known as Huw the Flitch, a handyman and gardener. He is the last of the original domestic staff to remain at the house. The former cook, Nancy, had left to live in Aberystwyth but is offered a substantial amount to come and resume her duties. With her comes her son Gwyn. He has never seen the valley before but knows everything about it, courtesy of his mother's stories. Someone else who knows a lot about the place is Huw, but he is very mysterious about it. Oddly, Nancy has told Gwyn nothing about Huw.

Alison hears scratching noises in the attic above her bed and persuades Gwyn to investigate. He finds stacks of dinner plates with a floral pattern. When he picks one up, something odd happens. He almost falls through the ceiling while simultaneously Roger, lounging by a large flat stone near the river, hears a scream and seems to see something flying through the air toward him. The stone is known as the Stone of Gronw. It has a hole neatly bored through it, and legend says that Lleu killed Gronw by throwing his spear clear through the stone that Gronw was holding to shield himself.

Alison begins behaving peculiarly. She traces the pattern on the plate onto paper and folds the result to make an owl. Finding out that Gwyn has been in the attic, Nancy demands that Alison give up the plate. Alison asserts she has no right to it, and eventually produces a blank white plate. She maintains that the pattern disappeared from the plate. Alison becomes obsessed by the plates. One by one she traces them and makes the owls, and one by one the plates become blank. The owls themselves disappear, though no one cares at first about some folded paper models.