Lanao del Sur , officially the Province of Lanao del Sur , is a province in the Philippines located in the Autonomous Region in Muslim Mindanao (ARMM). The capital is the city of Marawi, and it borders Lanao del Norte to the north, Bukidnon to the east, and Maguindanao and Cotabato to the south. To the southwest lies Illana Bay, an arm of the Moro Gulf.