The Green Girdle appears in the late 14th-century Middle English poem Sir Gawain and the Green Knight, composed by an anonymous poet in the West Midlands dialect and preserved in a single manuscript, Cotton Nero A.x, now in the British Library. The poem opens at Camelot on New Year's Day, where the Green Knight, a giant green man on a green horse, rides into Arthur's hall and offers a beheading game: he will submit to a single blow from any knight, provided that knight agrees to seek him out in a year and a day and receive the same blow in return. Gawain, Arthur's nephew and the youngest knight at the table, accepts the challenge, strikes off the Green Knight's head, and watches in horror as the headless body picks up the severed head, reminds Gawain of his promise, and rides away laughing. The girdle does not appear until the middle section of the poem, but its shadow falls across every line of Gawain's journey to the Green Chapel.

Gawain sets out a year later, armed and armoured, his shield painted with the pentangle, the five-pointed star of Solomon that symbolises his fivefold perfection: faultless in his five senses, faithful in his five fingers, devoted to the five wounds of Christ, strengthened by the five joys of Mary, and bound by the five virtues of friendship, fraternity, purity, politeness, and piety. On Christmas Eve, lost in the frozen wilderness of the Wirral Peninsula, he prays to the Virgin Mary and is led to a castle where he is received with lavish hospitality by its lord, Sir Bertilak de Hautdesert, a bluff and generous hunter, and by Bertilak's wife, a beautiful and clever woman who is far more dangerous than her husband's boar-hunts. Also present is an old woman, hideous and ancient, whom Gawain does not recognise as Morgan le Fay, Arthur's half-sister and the enchantress who engineered the entire beheading game to test the Round Table and frighten Guinevere to death.

Bertilak proposes a bargain: he will go hunting each day, and whatever he catches he will give to Gawain, on condition that Gawain give him in return whatever he may gain while the lord is away. Gawain agrees. For three mornings, while Bertilak hunts deer, boar, and fox in the winter woods, his wife visits Gawain's bedroom and attempts to seduce him. On the first two mornings, Gawain resists her advances with elaborate courtesy, allowing nothing but kisses, which he dutifully transfers to Bertilak each evening in exchange for the day's kill. On the third morning, the Lady changes her approach. She is more forward, more revealing in her dress, more direct in her language. When Gawain refuses her advances and declines a gold ring she offers, she presses upon him a green girdle of silk embroidered with gold thread, the same green as the Green Knight's skin and horse. She claims that any man who wears it cannot be slain by any cunning on earth, that it is charmed with protective magic.

Gawain is torn. He has resisted lust, but he cannot resist the fear of death. The next morning he must face the Green Knight's axe, and the girdle offers a way out. He accepts it, and the Lady makes him swear to keep it secret from her husband. That evening, when Bertilak returns with a fox, Gawain gives him the three kisses he received from the Lady but says nothing of the girdle. He has broken his word, violated the exchange agreement, and placed his trust in a magic charm rather than in God or his own virtue. The pentangle on his shield, the endless knot that cannot be broken, has been severed by a single strip of silk.

The next morning, Gawain rides to the Green Chapel with the girdle hidden beneath his armour, wrapped twice around his waist. The Green Knight is sharpening his axe on a grindstone, the sound echoing through the valley. He delivers three blows. The first two he holds back, explaining that they are for Gawain's truthful behaviour on the first two days of the exchange. The third draws a slight cut on Gawain's neck, and the Green Knight reveals why: the cut is for the girdle, for Gawain's failure to surrender it, for his cowardice in accepting it and his covetousness in hiding it. Then comes the greater revelation: the Green Knight is Bertilak, transformed by Morgan le Fay's magic, and the whole thing - the seduction, the hunt, the exchange, the girdle - was a test engineered by Morgan to expose the flaws in Arthur's court. Gawain, who had believed himself perfect, has been found wanting in only one small thing: he loved his life too much.

Gawain's reaction is one of the most human moments in medieval literature. He breaks down, flings the girdle at Bertilak's feet, and curses it as the token of his shame, his untruth, his cowardice, and his covetousness. He calls himself false and flawed, the worst of knights, and declares that he will wear the girdle forever as a reminder of his failure. But Bertilak refuses to accept this reading. He tells Gawain that he is absolved, that his confession is clean, that the penance has been paid by the point of the axe. He returns the girdle to Gawain and calls it a token of purity, a sign that Gawain has passed a test that no other knight could have endured. The two readings - shame and purity - hang in tension, and the poem refuses to resolve them.

When Gawain returns to Camelot and tells his story, the court's reaction is equally ambiguous. The knights and ladies laugh, not mockingly but with relief and affection, and every knight of the Round Table agrees to wear a green baldric, a sash across the chest, in honour of Gawain's adventure. The girdle has become an emblem of courtly brotherhood, a shared badge that acknowledges the universal frailty of even the best men. The poem ends with a prayer and a reference to "the thorn-crowned God," redirecting the reader's attention from the circular girdle to the circular Crown of Thorns, from Gawain's humiliation to Christ's triumph.

The connection between the Green Girdle and the Order of the Garter has fascinated scholars for generations. The Order of the Garter, founded by King Edward III in 1348 and inspired by Arthurian legend, is the oldest and most senior order of chivalry in Britain. Its motto, "Honi soit qui mal y pense" - Shamed be he who thinks evil of it - appears at the end of the Sir Gawain manuscript in a later hand, and some scholars have seen in Gawain's exclamation "corsed worth cowarddyse and couetyse boþe" a rough equivalent of the Order's spirit. In the parallel poem The Greene Knight, a late medieval retelling, the lace is white rather than green, and is considered the origin of the collar worn by the Knights of the Bath. Whether the poem influenced the Order or the Order influenced the poem, or both drew on a common tradition of girdle symbolism in chivalric culture, the Green Girdle remains the most famous sash in English literature.