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.
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.
In 1998, Mayor Vera Katz proposed capping a portion of the freeway in Downtown Portland with buildings and parks.