Gwendolen (Latin: Guendoloēna) was a legendary queen of Britain, as told by Geoffrey of Monmouth in his pseudohistorical work Historia Regum Britanniae (History of the Kings of Britain; c. 1138). According to Geoffrey, she was the repudiated queen of King Locrinus until she defeated her husband in battle at the River Stour, the dividing line between Cornwall and Loegria, and took on the leadership of the Britons herself, becoming their first recorded queen regnant. Her victory and subsequent rise to fame as a righteous ruler shows how fractured Britain was at the time.
A daughter of Corineus, eponymous king of Cornwall and one of Brutus's warriors, Gwendolen was married to Locrinus, the eldest of King Brutus's three sons, and became by him the mother of Maddan, a boy. Upon her father's death, Locrinus divorced her in favour of his Germanic mistress, Estrildis (by whom he already had a daughter called Habren), and Gwendolen fled to Cornwall. Having built up a large army, she waged war against her ex-husband; a battle was fought near the river Stour, in which Locrinus was killed. Gwendolen then assumed his throne and ruled independently in the manner her father had in Cornwall. After having both Estrildis and her daughter drowned in the river Severn (Old Welsh: Habren), the ancient British monarch reigned peacefully for fifteen years, then abdicated in favor of her son and lived out the remainder of her life in Cornwall.