Haidar Ali or Hyder Ali (both: hī´dər älē´), 1722–82, Indian ruler. A Muslim of peasant stock, he rose by military brilliance to command the army of the Hindu state of Mysore. By 1761 he was virtual ruler of Mysore and began expanding the dominions of that kingdom at the expense of the Maratha states and Hyderabad. In 1767 the British, in alliance with Hyderabad and the Marathas, took the field against Haidar. They were soon deserted by their allies, however, and Haidar, after some initial reverses, took his army to the outskirts of Madras (now Chennai) and dictated the peace (1769). Angered by the British refusal to honor a defensive alliance (formed in accordance with the 1769 peace terms) in 1772 and by their seizure of Mahé from the French in 1779, Haidar invaded the Carnatic in 1780 and routed a British force. In 1781 he was defeated near Madras (now Chennai) by Sir Eyre Coote. Haidar died a year later, but the war was continued by his son Tipu Sultan (see Tippoo Sahib).
See biography by N. K. Sinha (3d ed. 1959); study by L. B. Bowring (1969).