The Third Street Light Rail Project was the construction project that expanded the Muni Metro system in San Francisco, California, linking downtown San Francisco to the historically underserved southeastern neighborhoods of Bayview-Hunters Point and Visitacion Valley along the eastern side of the city.

In 1993, the San Francisco Municipal Railway published the Bayshore Transit Study, which offered the following seven goals:[2] The Bayshore Transit Study presented nine alternatives (including a "do nothing" alternative) ranging from building a trolley coach to expanding the MUNI Metro light rail system.

LRVs operate in mixed flow traffic at the 4th Street Bridge (the bridge does not have enough room for LRV exclusive right-of-way) and in a 10 block segment (from Kirkwood/La Salle stop through Revere/Shafter) in the Bayview business district to maintain parking on both sides of the street for customers of local businesses.

The Third Street Light Rail Project is the first part of a multi-phase plan to expand the Muni Metro system.

In addition, the second phase includes short turn service via the new Mission Bay Loop, which was completed in 2019[4] at a cost of $10.2 million.

In early March 2009, media and community groups proposed that as the Central Subway is being built, plans should be drawn up to extend the T Third past Chinatown through North Beach and Fisherman's Wharf neighborhoods, passing Pier 39, potentially using an old steam railroad line underneath Fort Mason and ending up at The Presidio.