The Falcon 9 rocket will carry Zuma, a US government spacecraft, into low-Earth orbit.
This launch has been delayed previously, in part due to weather, but the launch window opened again this evening at 5 p.m.
It's still unclear where the Zuma mission will be heading.
Originally scheduled to launch in November, problems with the rocket caused officials to delay plans by several weeks. (SpaceX has had 17 consecutive successful landings, so they're getting pretty good at this). SpaceX is expected to cut off its popular live stream early to hide the spacecraft's final destination.
The payload will be delivered to low Earth orbit, per the mission parameters, but we don't know anything else about its goal, design, or intended mission, because it's all classified.
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"As a company, Northrop Grumman realises this is a monumental responsibility and we have taken great care to ensure the most affordable and lowest risk scenario for Zuma", Mr Rains said, declining to comment on the nature of the payload.
The second stage continued carrying the mysterious Zuma to its destination in low-Earth orbit (LEO), while the first stage began maneuvering its way back to terra firma for a touchdown at Landing Zone 1, a SpaceX facility at Cape Canaveral.
SpaceX and the Pentagon did not respond to requests for comment about the nature of the mission.
It's also SpaceX's third such launch for the United States government, with previous payloads including a spy satellite and the X-37B space plane.