The mission - referred to as code name Zuma - took off from Cape Canaveral Air Force Station in Florida on a Falcon 9 rocket.
SpaceX is scheduled to launch an undisclosed national security payload on Sunday night.
A SpaceX Falcon 9 rocket carrying the secret Zuma spacecraft for the USA government launches Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida on January 7, 2018 in this still from a SpaceX video.
Originally planned for a November launch, the mysterious Zuma mission may finally go to space on Sunday evening.
Antonio Conte: What I like about new Chelsea signing Ross Barkley
Antonio Conte is quite pleased with the acquisition of Ross Barkley from Everton , hailing the midfielder as one with great potential.
Jon Gruden has $100 million reasons to coach the Raiders again
He was 2-2 in the post-season, losing in the AFC title game following the 2000 season and the divisional round in 2001. The Raiders have not identified their candidates for the head-coaching job.
Mega Millions winning ticket sold in Florida
Jackpot for the Friday night drawing has grown even bigger since nobody had the winning numbers in its Tuesday drawing. Most winners opt for cash options, which would be $281 million for Mega Millions and $358.5 million for Powerball.
SpaceX successfully sent something into space for the U.S. government, and that may be all we'll ever know about the so-called "Zuma" mission that Elon Musk's commercial space company completed from NASA's Kennedy Space Center in Florida Sunday evening.
The company's live webcast did not show video coverage of the Zuma spacecraft after it separated from the first stage of the rocket, but confirmed that the fairings deployed and the payload was well on its way to low-Earth orbit.
The company's first successful Falcon 9 rocket launch of 2018 was also probably the most secretive in the company's history.
It's also SpaceX's third such launch for the USA government, with previous payloads including a spy satellite and the X-37B space plane. SpaceX landed the rocket's first stage for reuse in a future launch, a key step in its goal to drive down the cost of access to space.