Locrinus was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He was the oldest son of Brutus and a descendent of the Trojans through Aeneas. He ruled a portion of Britain named Loegria, named after him, which is roughly the boundaries of modern-day England. He reigned 10 years, most of which were peaceful.

He revenged his brother, Albanactus's death by Humber the Hun by allying with his other brother, Kamber, and fighting Humber to the banks of a river where he drowned. The river was named Humber after this battle. Locrinus divided up the spoils of war with his allies, only keeping gold and silver found on their ships for himself. He also took the daughter of the king of the Germans, Estrildis, whom the Huns had captured. This angered Corineus, an ally of his father Brutus, who had arranged a marriage between Locrinus and his own daughter, Queen Gwendolen. Locrinus submitted and married Gwendolen but still secretly loved Estrildis, whom he locked in a cave beneath Trinovantum (London) for seven years.

Locrinus became the father of a girl, Habren, by Estrildis, and a boy, Maddan, by Gwendolen. Soon after Maddan's birth, Locrinus sent him off to Corineus, the child's grandfather. When Corineus finally died, Locrinus left Gwendolen and took Estrildis as his queen. Gwendolen went to Cornwall and assembled an army to harass Locrinus. The two armies met near the River Stour and there Locrinus was killed. His wife, Gwendolen, ruled after his death.