Gov. Jay Nixon is in favor of the proposed oil pipeline route through the middle of the United States, joining other Midwestern governors who are urging President Barack Obama to approve its construction.
The Democratic governor endorsed the Keystone XL pipeline in a letter to Secretary of State John Kerry on Monday.
"The approval and construction of the Keystone XL Pipeline will strengthen our economy, create jobs and promote North American energy independence," Nixon wrote.
The roughly 1,100-mile pipeline would carry oil derived from tar sands in western Canada to a hub in Nebraska, where it would connect with existing pipelines to carry the crude oil down to refineries in Texas. The new pipelines would not cross Missouri's borders.
The State Department is currently taking comments on the project before making a recommendation to Obama on whether to approve it. The department needs to sign off because the pipeline would cross a U.S. border.
Environmental groups have opposed the long-delayed project, arguing the pipeline would carry oil that contributes to global warming and could possibly spill.
Last month, the State Department reported no major environmental objections and Nixon dubbed the project as an "environmentally responsible way" to promote energy independence.
Nixon previously hadn't directly weighed in on the Keystone project, but did previously support the expansion of another oil pipeline in Missouri. Last year, he praised a Canadian company's announcement to build a new 600-mile pipeline from Illinois to Oklahoma along an existing pipeline route running diagonally from northeast to west-central Missouri.
Missouri's two U.S. senators - Democrat Claire McCaskill and Republican Roy Blunt - have also endorsed construction of the Keystone pipeline.
Obama told a group of state governors at a White House meeting in February that he expects to make a decision on the project's future in a couple of months.