The Isthmus of Ofqui is a narrow isthmus that connects the Taitao Peninsula with the Chilean mainland. The isthmus is bounded in the south by the Gulf of Penas, in the north by the San Rafael Lagoon, in the west by the Taitao Peninsula and in the east by the Northern Patagonia Ice Field.
Much of the swampy isthmus is drained by San Tadeo River that flows in SSW direction to the Gulf of San Esteban.