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. 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 1,800 to 2,100 miles (2,900 to 3,400 km) of roadbeds of the Oregon Trail in the 1840s–1880s.

After the reports of the Lewis and Clark Expedition were published about 1807, a small steadily increasing stream of isolated pioneer groups began settling the valley and improving the explored road from the east set up by the fur traders and mountain men as they came. From the 1841 trail opening, when the effort of many over many years finally widened the fur traders' mule trails into an improved rough road just capable of carrying the width of a wagon, settlers charged into the region along the new trail, creating new settlements centered about colonial Oregon City as the early capital, even before ownership of the region was settled. So many came, the valley led the way to achieving statehood in less than 16 years after its ownership was settled on the United States in 1846. A small part of the Willamette Valley ecoregion is in southwestern Washington around the city of Vancouver, which was once the site of an early colonial-era settlement—Fort Vancouver. The Willamette Valley—served with its saw mills, lush productive farms, handy river transport network, and nearby timber and mineral resources—developed naturally as a cultural and major commercial hub, as the Oregon Country became the Oregon Territory. The valley forms the cultural and political heart of Oregon and is currently home to 70% of Oregon's population.