Eustace Clarence Scrubb is a fictional character in C. S. Lewis's Chronicles of Narnia. He appears in The Voyage of the Dawn Treader, The Silver Chair, and The Last Battle. In The Voyage of the Dawn Treader, he is accompanied by Edmund and Lucy Pevensie, his cousins. In The Silver Chair and The Last Battle, he is accompanied by Jill Pole, a classmate from his school.

Eustace is portrayed at first as arrogant, whiny, and self-centered. It can be gathered from Eustace's behavior, and the tone that Lewis used in describing his family and school, that Lewis thought such behavior silly and disliked it a great deal. In fact, at the beginning of The Voyage of the Dawn Treader, Lucy and Edmund dislike visiting him and his parents, though that has mostly to do with Eustace's arrogant and unfriendly attitude, and he also calls his parents by their first names. However, the book deals with his rehabilitation (just as The Lion, the Witch and the Wardrobe dealt with Edmund's) and in the later books, he is an altogether better person, becoming a hero along with his friend Jill Pole, although he still sometimes tends to be a know-it-all. It is mentioned in the Silver Chair that Eustace is afraid of heights, causing him to overreact when Jill goes too close to the edge of a cliff. In trying to stop her he falls. In other respects, Eustace displays great courage and a fair degree of discernment in facing the challenges that confront him in the Narnian world.

According to Lewis's Narnian timeline, Eustace was born in 1933 and is 9 years old when he appears in The Voyage of the Dawn Treader; and by the time of The Last Battle he is 16 years old.

Eustace is introduced at the beginning of The Voyage of the Dawn Treader with the opening line, "There was a boy called Eustace Clarence Scrubb, and he almost deserved it." He is the only child of what Lewis describes as "very up-to-date and advanced people," who send him to a progressive mixed school. Eustace calls his parents by their first names (Harold and Alberta); his parents are vegetarians, nonsmokers, teetotallers, pacifists, and wear an unspecified special kind of underclothes.

Much of the narrative of The Voyage of the Dawn Treader concerns the personal growth of Eustace, as he is drawn into Narnia and aboard the eponymous ship along with Edmund and Lucy, and into adventures that bring him to realise how self-centred his attitudes are. Part of the story is told with extracts from his diary, where we see how skewed his point of view is. He describes the ship sailing in perpetual storm (though the weather is fine), and portrays the others as foolishly denying the supposed rough seas and refusing to face the "truth" of the situation. He complains when Lucy is given Caspian's cabin, and comments to the crew that giving girls special treatment is actually "putting them down, and making them weaker". Moreover, he cannot accept that he is in the Narnian world: he imagines that he can "lodge a disposition" (or "bring an action") at a British consulate or a British court; and he is beaten by Reepicheep for treating the mouse as one might a circus animal.

Eustace wanders off by himself when the ship puts ashore on an unexplored island. He falls asleep on a dead dragon's hoard and finds himself transformed into a dragon by "greedy, dragonish thoughts" in his heart. He also finds himself in constant pain from Lord Octesian's arm bracelet, which he put on as a boy but is too small for a dragon's leg. Upon return to the Dawn Treader, he is nearly attacked by the crew until Lucy asks if he is Eustace, to which he vigorously nods his head. Being a dragon changes Eustace; instead of behaving like his usual sulky self, he helps the travellers find food, shelter, and a tree to serve as a new mainmast. The problem comes when it is time to leave the island, as the ship cannot hold or maintain a dragon. Reepicheep displays sympathy to Eustace's plight despite the boy's prior cruelty to the mouse and they eventually become friends.

Eventually, Eustace meets Aslan, who returns him to human form by peeling off his dragon skin and sending him into a refreshing bath. Edmund shares his own redemption story with Eustace, observing that "you were only an ass, but I was a traitor." Eustace improves after this, and becomes a valuable member of the expedition. When the ship is in danger of being sunk by a giant sea-serpent, Eustace attacks the monster, using only a sword. When Eustace returns home after his adventures, his mother thinks he has become tiresome and commonplace, blaming the change on the influence of "those Pevensie children" — though everyone else thinks that he has become a much better person.

Eustace returns to his progressive school where he is now labeled a misfit, due to the changes in him wrought during The Voyage of The Dawn Treader. Whereas previously he was a crony and tale-bearer to the gang of bullies who are given free rein at the school, he is now one of their targets, but has the courage to withstand them. He befriends fellow misfit Jill Pole, and their desire to leave the school draws them into Narnia. This unlikely friendship (given that Eustace had bullied Jill before his experience in Narnia) is strengthened throughout the story. Following the custom of their school, Eustace and Jill address each other by their surnames, "Scrubb" and "Pole".