Agravain or Agravaine (/ˈæɡrəveɪn/ AG-rə-vain) is a Knight of the Round Table in Arthurian legend, whose first known appearance is in the works of Chrétien de Troyes. He is the second eldest son of King Lot of Orkney with one of King Arthur's sisters known as Anna or Morgause, thus nephew of King Arthur, and brother to Sir Gawain, Gaheris, and Gareth, as well as half-brother to Mordred. Agravain secretly makes attempts on the life of his hated brother Gaheris starting in the Vulgate Cycle, participates in the slayings of Lamorak and Palamedes in the Post-Vulgate Cycle, and murders Dinadan in the Prose Tristan. In the French prose cycle tradition included in Thomas Malory's Le Morte d'Arthur, together with Mordred, he then plays a leading role by exposing his aunt Guinevere's affair with Lancelot, which leads to his death at Lancelot's hand.

In the traditional, albeit contested, division of the massive medieval prose Lancelot portion of the Vulgate Cycle into three or four parts, the last section is named after Agravain. Despite giving his name to the section, Agravain plays only a minor part in most of its stories.

The earliest known appearance of Agravain, as Engrevain the Proud (Old French: li Orgueilleus, modern French: l'Orgueilleux), is found in Chrétien de Troyes' 12th-century romance poem Perceval, the Story of the Grail in which he is one of Gawain's brothers and is also known as the one "with the hard hands" (aus dures mains). The poem's anonymous First Continuation describes him as very quarrelsome. In Sir Gawain and the Green Knight, where he is called Agravain of the Hard Hand, he is named in a list of respectable knights. This, combined with his unobjectionable depiction in Chrétien's original Perceval, suggests his reputation might not have been very negative prior to his characterisation in the prose cycles.

In the Lancelot-Grail (also known as the Vulgate Cycle) prose works, Agravain is generally portrayed as a handsome man, taller than Gawain, and a skilled fighter. However, unlike his heroic brothers Gawain and Gareth, Agravain is known for malice and villainy, yet sometimes capable of heroic deeds. In the Prose Lancelot part of the Vulgate Cycle, Agravain is described as taller than Gawain and with a "somewhat misshapen" body. As "a fine knight" but "arrogant and full of evil words [and] jealous of all other men," he "was without pity or love and had no good qualities, save for his beauty, his chivalry [knightly values], and his quick tongue."

In Jean Froissart's Méliador, Agravain courts and marries Florée, a cousin of Princess Hermondine of Scotland, after winning her tournament at Camelot. In Thomas Malory's Le Morte d'Arthur, Arthur marries him to Laurel, a niece of Lynette and Lyonesse.

A major motif regarding Agravain's character in the prose romances is his one-sided conflict with his younger brother, Gaheris, in addition to his rivalry with Gawain. According to the Vulgate Merlin, Gawain and his two full brothers came to court together as squires and were knighted together. When Agravain brags to his brothers that he would make love to an unwilling damsel if he wanted, Gaheris responds with mockery, and Agravain attacks him, only to be knocked down by Gawain, who admonishes Agravain for his proud ways and bullying nature. In the Post-Vulgate Cycle retelling, Gaheris is ordered by Merlin to seek out and free Gawain from captivity. Feeling that Merlin always unfairly favoured Gaheris, Agravain is very jealous and declares that he could rescue Gawain just as good or better than he, yet it is Gawain who achieves the quest. A prophecy says that Gaheris must be knighted first and then he should knight his brothers, however Agravain still insists that he must be knighted only by King Arthur himself, relying on his age. He then follows secretly his younger brother, who set out on a quest, determined to prove that he is a better knight than Gaheris and to once and for all settle this issue by cutting his brother's head off. Yet Gaheris defeats the incognito Agravain twice (including still beating up his attacker in an ambush while unprepared and weary from an earlier fight), failing to learn his mysterious opponent's true identity in the process but nevertheless making Agravain stop trying to kill him by making clear he is in fact vastly superior to him. Years later, upon learning that Gaheris has murdered their mother, Morgause, Gawain swears to avenge her. Agravain, for though he had loved his mother, hated Gaheris more and so was glad to see that his brother had done such a deed for which he hoped to see Gaheris put to death. But when Agravain and his half-brother Mordred are at the point of beheading Gaheris, Gawain stops them as he believes that they should not shame themselves by killing one who was their brother. The four later attack Morgause's lover Lamorak, and they kill him after an unfair fight of all of them at once against one.

The so-called "Agravain" section of the Vulgate Cycle's Prose Lancelot begins with some minor adventures of Agravain. In one of them, he slays the evil lord Druas the Cruel. The Prose Lancelot ascribes an important adventure of Lancelot, which is here retold in the order in which it is supposed to have occurred rather than the textual order which includes explanations told by Agravain at the end. It tells of Agravain being cursed by two damsels on separate occasions, one for wounding a knight in his arm and then joking about it and another for trying to force himself on her and then commenting on seeing her infected leg. Later, he learns that his love, the daughter of King Tradelmant of North Wales, is seeking for him to rescue her, for her father has bestowed her on a knight whom she does not want to marry. Agravain manages to win her for himself and joins the Duke of Cambenic, who gives him a castle. He then lives there with her and with his young half-brother Mordred, who at that time is still a squire. But a curse affects Agravain's left arm and the other his left leg, leaving him to greatly suffer until these limbs are anointed with the blood of the best knight alive as well as of the second-best. They decide to send for Gawain but also to seek out the mysterious Black Knight (the incognito Lancelot) that saved Arthur's throne from Galehaut. A messenger brings Gawain, who agrees to give blood that heals Agravain's leg, showing that Gawain is the second-best knight alive. Gawain then finds and persuades Lancelot to give his blood, which does its job, proving that Lancelot is indeed the best knight in the world.

In the Post-Vulgate Grail Quest, Agravain and Gawain (the latter villainized within the Post-Vulgate Cycle compared to his usual portrayals) come upon wounded Palamedes. Palamedes protests that he is now a Knight of the Round Table like them and so they should not fight him, but Gawain cares nothing of their Pentecostal Oath and attacks, joined by Agravain. However, when their opponent is beaten down to near death, Agravain asks Gawain to hold back, which is the only time within the cyclic prose romances when he shows compassion. When Gawain refuses to listen and beheads Palamedes anyway, Agravain says he is grieved because Palamedes was such a good knight and, more practically, because this deed will be hard to conceal. In the Prose Tristan, after the end of the Grail Quest, Agravain and Mordred, who both hate Dinadan, see him coming wounded outside Camelot and decide it as a good time to take vengeance, as Arthur's court believes that Dinadan is still in Cornwall. Dinadan manages to fight them off, but they return to attack him again within the sight of Camelot. Dinadan is now too weak to stand up to both of them, and so Mordred quickly knocks him from his horse, and Agravain finishes him off. Lying, they later claim the dying Dinadan was mistaken in blaming them for the attack, and it must have been some other knights who murdered him. In Malory's telling, Agravain also insists on fighting Tristan together with Gaheris. In this combat, Tristan severely wounds Agravain and calls the Orkney brothers (sans Gareth) the most notorious murderers of good knights in Arthur's realm.