Interstate 405 (I-405, also known as the Stadium Freeway No. 61) is a short Interstate Highway in Portland, Oregon. It forms a loop from Interstate 5 that travels through downtown Portland west of the Willamette River.

Interstate 405's southern terminus is at a complex Y-like stack interchange with Interstate 5 and Harbor Dr. (formerly Harbor Freeway) in the South Portland neighborhood, near the southern edge of downtown. The highway travels northwest for about a mile, passing the campus of Portland State University, to an interchange with the U.S. Route 26 freeway bound for the western suburbs of Portland. I-405 then travels north for just over a mile, forming the western edge of Downtown Portland. It runs mostly below grade through this area. At the north edge of downtown, as it approaches the Willamette River, I-405 reaches a Y interchange with U.S. Route 30. This interchange and the short freeway portion of U.S. 30 to which it connects were originally intended for the canceled I-505.

I-405 and U.S. 30 share alignment as they cross the Willamette River on the double-decker Fremont Bridge. At the northeastern end of the bridge is a partially completed stack interchange with Interstate 5; the canceled Rose City Freeway was to be a continuation of I-405, serving Portland International Airport and northeast Portland. Only a small stub portion of the Rose City Freeway was built, in order to provide a connection between the bridge and nearby Legacy Emanuel Hospital.

The earliest portion of Interstate 405 opened to traffic on February 25, 1969. The loop was completed in 1973 when the Fremont Bridge opened, linking I-405 to I-5 in North Portland. In August 2009 the entire route with the exception of the Fremont bridge was completely repaved for the first time after it opened to traffic. The bridge itself was repaved in August 2011.

The entire route is in Portland, Multnomah County.