Wallula Gap (/wəˈluːlʌ ɡæp/) is a large water gap of the Columbia River in the northwest United States in southeast Washington. It cuts through the Horse Heaven Hills basalt anticlines in the Columbia River Basin, just south of the confluence of the Walla Walla and Columbia rivers. It has been recognized as a National Natural Landmark by the National Park Service as a site that provides an important illustration of the geological history of the United States.