Ilocos Sur (lit. 'South Ilocos'), officially the Province of Ilocos Sur (Ilocano: Probinsia ti Ilocos Sur; Tagalog: Lalawigan ng Ilocos Sur), is a province in the Philippines located in the Ilocos Region in Luzon. Located on the mouth of the Mestizo River is the capital of Vigan, while Candon is the most-populous city. Ilocos Sur is bordered by Ilocos Norte and Abra to the north, Mountain Province to the east, La Union and Benguet to the south and the South China Sea to the west.
Ilocos Sur was established in 1818 when the province of Ilocos was split into two: the north (now Ilocos Norte) and the south (Ilocos Sur). At that time, it included parts of Abra and the upper half of present-day La Unión. The current boundaries of the province set in March 1917 by virtue of Act No. 2683.