The Willamette Valley ( /wɨˈlæmɨt/ is the most populated region in the state of Oregon of the United States. Located in the state's northwest, the region is surrounded by tall mountain ranges to the east, west and south and the valley's floor is broad, flat and fertile because of Ice Age conditions. Located centrally inside the large alluvial-deposited soils of the Willamette River drainage basin, the valley spreads far from the river banks to both the east and west barrier ranges as the river proceeds northward from its emergence from the Calapooya Mountains near Eugene to the confluence of the Willamette with the Columbia River at Portland. The valley's waterways and tributary streams and valleys were of great importance for water transport in the development of the Oregon Territory and young state until well past the arrival of modern roads and highways.

The valley holds several of the state's principal cities. A majority of the state's population lives within the basin, where one or more massive Ice Age floods left the valley floor thick with flood-carried sediments, making the valley extremely fertile. A massively productive agricultural area, the valley was widely publicized from the 1820s as a 'promised land' of the 'flowing milk and honey' sort and became, at Oregon City, the destination of choice for the oxen-drawn wagon trains of organized emigrants traveling west on the perilous and rough 1800–2100 miles roadbeds of the Oregon Trail in the 1840s–1880s.