Santiago is a 5th class municipality located in the central coast of Ilocos Sur Province. It is bounded on the north
by San Esteban, on the east by Burgos, Lidlidda, and Banayoyo, on the south by Candon City, and on the west by South China Sea.
It has a total land area of 74.5 square kilometers and is politically subdivided into 24 barangays.
The town is situated 358 kilometers north of Manila and 49 kilometers south of Vigan City, the provincial capital.