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.

Sound Transit (ST), officially the Central Puget Sound Regional Transit Authority, is a public transit agency serving the Seattle metropolitan area in the U.S. state of Washington. It operates the Link light rail system in Seattle and Tacoma, regional Sounder commuter rail, and Sound Transit Express bus service. The agency also coordinates the regional ORCA fare card system, which is also used by local transit operators. In 2019, Sound Transit services carried a total of 48 million passengers and averaged over 161,000 riders on weekdays.