Sound Transit 3, abbreviated as ST3, was a ballot measure during the November 2016 elections in Seattle, Washington, proposing an expansion of the regional public transit system. The measure was proposed by Sound Transit, which was established by a similar initiative passed in 1996 and expanded by the Sound Transit 2 vote in 2008, who have operated regional transit systems in the Seattle metropolitan area since 1999. On November 8, 2016, Sound Transit 3 was approved by over 54 percent of voters in the Puget Sound region; voters in Pierce County rejected the measure, but the measure passed in King and Snohomish counties, and had an overall majority.

The $53.8 billion Sound Transit 3 plan will expand the existing Link light rail system to the suburbs of Tacoma, Federal Way, Everett and Issaquah, as well as the Seattle neighborhoods of Ballard and West Seattle. The local portion of the measure would be partially funded by increases in sales tax, motor vehicle excise tax, and property tax. The use of the motor vehicle tax, which included an older depreciation scale for vehicles, became a source of major controversy after the measure was passed.

The resulting transit network after the completion of Sound Transit 3 is planned to include 62 miles (100 km) of additional light rail serving 37 new stations; the entire 116-mile (187 km) light rail system would carry an estimated 600,000 daily passengers. A Sounder commuter rail extension to DuPont and bus rapid transit lines on State Route 522 and Interstate 405 are also part of the package. The package's projects are set to open in stages from 2024 to 2041, with light rail construction beginning in the late 2020s for most extensions.

Sound Transit, officially the Central Puget Sound Regional Transit Authority, was established in 1993 to build a regional mass transit system pending approval from voters. After an unsuccessful ballot measure in 1995, the "Sound Move" plan was approved on November 5, 1996, financing the construction and operation of a $3.9 billion light rail, commuter rail and express bus system. The services would begin service over the next decade, beginning with Sound Transit Express buses in 1999 and followed by Sounder commuter rail to Tacoma in 2000 and Everett in 2003. Light rail service came to Tacoma in 2003 and the centerpiece of the plan, Central Link from Downtown Seattle to Seattle–Tacoma International Airport, opened in 2009. The development of light rail was hindered by a funding crisis in the early 2000s, which forced the plan to be scaled back from 23 miles (37 km) to 16 miles (26 km), with planning for completion of the rest of the system. The final project funded by Sound Move, the University Link light rail extension, opened in 2016.

A second ballot measure, called Sound Transit 2, was proposed in 2007 to fund the deferred light rail segments left off the scaled-back system and further expand into the suburbs. It went to the ballot in November 2007 as part of a joint "Roads and Transit" measure proposing $47 billion in regional transportation projects, of which $30.8 billion would be used to build 50 miles (80 km) of light rail towards Lynnwood to the north, Redmond to the east, and Tacoma to the south. The proposition was rejected, with the opposition campaign led by environmentalists (including the Sierra Club and King County Executive Ron Sims) who were concerned about the 186 miles (299 km) of new highway lanes and ramps that would have been built by the plan. A standalone and scaled-back transit proposition was passed the following year, approving $17.9 billion for 34 miles (55 km) of light rail towards Lynnwood Transit Center, the Microsoft campus in southwestern Redmond, and northern Federal Way, as well as additional trips on Sounder.

During the implementation of Sound Transit 2, the effects of the Great Recession on sales tax revenue forced Sound Transit to scale back its projects in 2010. Light rail to Federal Way was truncated to South 200th Street in SeaTac, while the rest would be studied for a future expansion; East Link to Bellevue and Redmond was delayed to 2023 from 2021. The majority of Sound Transit 2's light rail projects are scheduled to be completed in the early 2020s, although Lynnwood Link and East Link were both delayed due to construction issues and increased costs.

Planning for a third phase of transit expansion began in 2008 with $82 million in appropriated funding from Sound Transit 2 for "ST3 planning". Sound Transit identified several corridors to receive comprehensive planning studies, including Lynnwood to Everett, South Bellevue to Issaquah, Redmond to the University District via Kirkland, University District to Ballard to Downtown Seattle, Burien to West Seattle to Downtown Seattle, Burien to Renton, and bus rapid transit on Interstate 405. One of the studies, the Ballard–Downtown Seattle corridor, was made into a joint study with the city government of Seattle in 2013 that would investigate light rail as well as rapid streetcar concepts as part of the municipal streetcar network. Other high-capacity transit corridor studies and an updated long-range plan were approved by the Sound Transit Board in February 2013, as part of a requirement for a potential public vote for a next phase. The updated long-range plan, released in December 2014, recommended the addition of light rail extensions to Paine Field in Everett, West Seattle, Issaquah, and Tacoma Mall to the 2005 plan, which already included funded lines and light rail to Everett, Tacoma and Ballard.

Sound Transit presented a $15 billion transit package, named "Sound Transit 3", in October 2014, including light rail expansions to Everett, Tacoma, Issaquah, Ballard, and West Seattle. The following month, the Sound Transit Board unanimously voted to pursue new taxes to support the 2016 ballot measure, pending authorization from the Washington State Legislature. While the Republican-majority state senate attempted to the limit the package to a maximum of $11 billion, authorization for the full $15 billion revenue cap (for the first 15 years of collection) was passed in July 2015 as part of a statewide transportation package. As part of a compromise, a sales tax of 3.25 percent would be charged by the state on materials for construction projects, raising $518 million in revenue for the state highway funds that would be appropriated for other programs.