In the U.S. state of Oregon, U.S. Route 26 is a major cross-state state highway, connecting U.S. Route 101 on the Oregon Coast near Seaside with the Idaho state line east of Nyssa. Local highway names (see Oregon highways and routes) include the Sunset Highway No. 47, Mount Hood Highway No. 26, and John Day Highway No. 5.

The westernmost segment, known as the Sunset Highway No. 47, heads southeast from US 101 near Seaside to Portland. (Prior to 2004, its origin was near Astoria and continued south to Seaside concurrent with US 101.) The Sunset Highway becomes a freeway near Banks, serving Washington County suburbs of Portland such as North Plains, Hillsboro, and Beaverton.

In Portland, the route overlaps Interstate 405 (Stadium Freeway No. 61) for a short distance before exiting onto city streets, including Arthur Street, to reach the Ross Island Bridge. (Prior to 2005, US 26 ran through downtown Portland on the one-way couplet of Market and Clay Streets, which carry the Sunset Highway to its end at Naito Parkway (Pacific Highway West No. 1W), turning south there to reach the bridge.) US 26 leaves the bridge, which is at the beginning of the Mount Hood Highway No. 26, and follows Powell Boulevard, a surface street, to Gresham.

There were plans to construct a freeway alignment of US 26 â€” the Mount Hood Freeway â€” to bypass Powell Boulevard; however this project was canceled as a result of the freeway revolts in the 1970s. A few ramp stubs from Interstate 5 (on the Marquam Bridge) stand as evidence of this project. Roadway connections between the Portland freeway network and Mount Hood remain a big problem, as there is no good direct highway connection. The proposed Sunrise Corridor may eventually offer a long-term solution; however construction of this route is several years off.

An expressway begins near Gresham and carries US 26 southeast to near Sandy. From Sandy to near Government Camp and Bennett Pass, where US 26 intersects Oregon Route 35, it closely follows the historic Barlow Road through the Mount Hood Corridor, and is part of the Mount Hood Scenic Byway. The Mount Hood Highway branches off to the north along OR 35, and the Warm Springs Highway No. 53 carried US 26 southeast through Blue Box Pass, the Warm Springs Indian Reservation, and Agency Plains to Madras. After a short overlap with US 97 (The Dalles-California Highway No. 4), the short Madras-Prineville Highway No. 360 continues southeast to a junction with OR 126 in Prineville.

At that junction, US 26 picks up the Ochoco Highway No. 41, which also follows OR 126 west to US 97 in Redmond. The Ochoco Highway ends at OR 19 near Dayville, from which US 26 follows the John Day Highway No. 5 through John Day to US 20 in Vale. The remainder of US 26 in Oregon overlaps US 20 on the Central Oregon Highway No. 7 to the Idaho state line.

An ancient trail passed through the section of the Warm Springs Indian Reservation as part of an extensive Indian trade network linking peoples of the northern Great Basin and Columbia Plateau to those living west of the Cascade Range. Obsidian, bear grass, and slaves were transported over these trails to major trading locations along the Columbia River in exchange for dried salmon, smelt, sturgeon, and decorative sea shells. The long established route was later used by Peter Skene Ogden's fur trapping expeditions in 1825 and 1826. Fur trader Nathaniel Wyeth was here in the 1830s. Captain John C. Frémont followed this route on his 1843 explorations for the United States and Lieutenant Henry Larcom Abbot headed a Pacific Railroad survey party along it in 1855.