The Lewis and Clark Centennial Exposition, commonly also known as the Lewis and Clark Exposition, and officially known as the Lewis and Clark Centennial and American Pacific Exposition and Oriental Fair, was a worldwide exposition held in Portland, Oregon, United States in 1905 to celebrate the centennial of the Lewis and Clark Expedition. While not officially considered a World's Fair by the Bureau of International Expositions, it is often informally described as such; the exposition attracted both exhibits and visitors from around the world. During the exposition's four-month run, it attracted over 1.6 million visitors, and featured exhibits from 21 countries. Portland grew from 161,000 to 270,000 residents between 1905 and 1910, a spurt that has been attributed to the exposition.

Portland is a port city in the Pacific Northwest and the most populous city in the U.S. state of Oregon. Situated in the northwestern area of the state at the confluence of the Willamette and Columbia rivers, it is the county seat of Multnomah County, Oregon's most populous county. As of 2020, Portland's population was 652,503, making it the 26th-most populous city in the United States, the sixth-most populous on the West Coast, and the second-most populous in the Pacific Northwest, after Seattle. Approximately 2.5 million people live in the Portland–Vancouver–Hillsboro, OR–WA metropolitan statistical area, making it the 25th most populous in the United States. About half of Oregon's population resides within the Portland metropolitan area.