The MAX Yellow Line is a light rail line serving Portland, Oregon, United States. Operated by TriMet as part of MAX Light Rail, it connects North Portland, Portland City Center, and Portland State University (PSU). The line serves 17 stations; it runs north–south from Expo Center station to PSU South/Southwest 6th and College station, interlining with the Green and Orange lines within the Portland Transit Mall. Service runs for 21 hours per day with headways of up to 15 minutes. The Yellow Line is the fourth-busiest service in the MAX system; it carried an average 12,960 riders per weekday in September 2019.

After failing to secure funding for a planned light rail line between Clackamas County and Clark County, Washington called the South/North Corridor, Portland business leaders and residents convinced TriMet to revive a portion of the project within North Portland along the median of Interstate Avenue. The ten-station, 5.8-mile (9.3 km) Interstate MAX extension began construction in 2001 and opened to Yellow Line service on May 1, 2004. From its opening until 2009, the Yellow Line ran from Expo Center station in North Portland to the Library and Galleria stations in downtown Portland. In 2009, TriMet rerouted downtown Yellow Line service to the Portland Transit Mall.

Since 2015, the Yellow Line has operated as a northbound through service of the Orange Line from PSU South/Southwest 6th and College station. Conversely, most southbound Yellow Line trains, which had served the other half of the transit mall on 5th Avenue from 2009 to 2015, operate through to the Orange Line from Union Station/Northwest 5th & Glisan station.

Proposals for a light rail line through North Portland, across the Columbia River, and into Vancouver, Washington, were considered as early as the 1980s. A study by staff of the Portland metropolitan area's regional government, Metro, in 1985 examined the feasibility of a line alongside Interstate 5 (I-5) or along the median of Interstate Avenue but concluded that no light rail alternative would "'pay back' within the useful life of the project". A different report completed in 1986, however, noted that light rail along the corridor would be "promising". In 1988, Portland city planners proposed a northside rail service as part of Portland's Central City and Albina Community plans; they sought to extend the region's then-two-year-old light rail system, the Metropolitan Area Express (MAX), via Interstate Avenue, I-5, or Martin Luther King Jr. Boulevard (formerly Union Avenue). While serving on the Senate Committee on Appropriations, U.S. Senators Mark Hatfield of Oregon and Brock Adams of Washington combined this proposal with a greater Vancouver–Portland–Oregon City light rail plan that Metro separately developed, for which the committee appropriated $2 million to study in 1989.

Preliminary alignment studies north to Vancouver and Clark County, including an additional proposal for a line between Vancouver Mall and Clackamas Town Center along I-205, commenced shortly after. Metro's Joint Policy Advisory Committee on Transportation (JPACT) identified a 25-mile (40 km) route from Hazel Dell through downtown Portland to Clackamas Town Center in 1994 that TriMet formally named the "South/North Corridor". That November, Metro asked Portland area voters if they would approve a $475 million bond measure to cover Oregon's portion of the project's estimated $2.8 billion cost; the measure passed by 63 percent. Across the river, Clark County officials proposed a 0.3 percent increase in sales and vehicle excise taxes to provide Washington's $237.5 million share; voters turned it down by 69 percent on February 7, 1995.

Amid fears that ridership would not justify a North Portland segment if Clark County were excluded, JPACT scaled back the project and released a second plan that would only build the line between the Rose Quarter and Clackamas Town Center. To fill the funding gap that resulted from the exclusion of Clark County, the Oregon House of Representatives passed a $750 million transportation package that included $375 million for the project. The Oregon Supreme Court promptly struck down this funding due to the inclusion of unrelated measures, which violated the state's constitution. In February 1996, state legislators revised the package, but light rail opponents forced a statewide vote in November that ultimately prevented the use of state funds. In an effort to regain the support of North Portland residents, who had historically voted in favor of light rail, and to avoid seeking state funding, JPACT announced a third plan in February 1997 that reinstated a segment within North Portland, a 15-mile (24 km) line from Lombard Street to Clackamas Town Center. A few months later, the Portland City Council extended this proposed alignment through North Portland so that it would terminate another mile north of Lombard Street in Kenton. That July, Metro advanced the final environmental studies for a line that would run 16 miles (26 km) between Kenton and Clackamas Town Center in its first phase, with a potential to extend it 21 miles (34 km) up to Clark County should financing be acquired. Due to the wording on the original ballot passed in 1994, which described the project extending into Clark County, regional transit agency TriMet elected to reaffirm voter support by drafting a new $475 million bond measure. Portland area residents cast their vote on November 3, 1998, and those against the measure narrowly defeated it, 52 percent to 48 percent.

In 1999, North Portland residents and city business leaders urged TriMet to revive the South/North Corridor's northern portion but without the Clark County segment; they argued that 81 percent of Multnomah County voters had wanted light rail. TriMet agreed and developed a proposal to build a line along the median of Interstate Avenue, between the Portland Expo Center and the Rose Quarter. Meetings and polls conducted in June of that year determined that locals overwhelmingly supported the project, which organizers began calling the "Interstate MAX", as long as it was less expensive than the South/North project, did not displace residents from their homes, and did not require any new taxes. The city council subsequently endorsed the proposal.

TriMet projected the cost of the Interstate MAX at $350 million. To build it without the need for a significant new source of local funding, the city created an urban renewal district surrounding the alignment and adopted the Interstate Corridor urban renewal area (ICURA) plan in August 2000. This covered an expansive 3,744-acre (1,515 ha) area within 10 neighborhoods and directed $30 million in tax increment funds towards the project. That same year, TriMet and the city completed funding the Airport MAX and Central City Streetcar projects without requesting any federal assistance; TriMet declared them part of the Interstate MAX project, providing $257.5 million in matching federal funds that the Federal Transit Administration approved in September. TriMet and Metro contributed $38.5 million and $24 million respectively to the remaining balance, sourced from their own general transportation funds.