A 23-storey Falcon 9 rocket carrying the NROL-76 for the national reconnaissance office left the launchpad at 7:15am local time today from the Kennedy Space Center in Florida.
The launch was scheduled to take place on Sunday, but lift-off was cancelled at the last minute by a bad sensor.
SpaceX cut off launch coverage two minutes and 48 seconds after liftoff at the request of the NRO, shortly after the second-stage engine fired up to boost the spy satellite into a low-Earth orbit.
In March, SpaceX successfully re-launched a used booster - or "flight proven", in SpaceX terminology - for the first time for a commercial satellite mission. "The certification process needs to properly account for previously flown hardware to launch our national security space systems safely and securely".
"And we have touchdown", a SpaceX commentator said on a live video as cheers broke out at mission control. But the upper-level wind at liftoff was "unusually high".
It was the company's fourth zone landing and the 10th overall successful landing of the first stage booster.
"I can tell you that everyone of our launches, everyone of our payloads we put up in space brings new, awesome technology, new capabilities to our warfighters, our policy makers, all of it helps keep us safe as a nation", said Karen Furgerson, of the NRO.
The classified payload for this morning's NROL-76 launch is suspected to be a data relay satellite, according to Space News.
The larger portion of the rocket, known as the first stage, made a gentle arc and powered its nitrogen thrusters to guide it back to Earth. The weather seemed to cooperate as there were no further delays on the top secret SpaceX SpySat launch.
For the past ten years the USA military have carried out space launches with United Launch Alliance which is a partnership of Lockheed Martin and Boeing, a monopoly Musk has battled for years to break.
The launch was SpaceX's fifth this year, continuing its rebound from a launch pad rocket explosion last summer that grounded the company for the last four months of 2016.
Despite a few weather hiccups, Monday's launch was deemed successful.