The rocket successfully launched at 11:50 pm (0350 GMT on Friday), according to SpaceX's livestream of the event.
The Falcon 9 lit up the night sky over NASA's Kennedy Space Center, where it was launched from.
The US Space Force has said the drone's mission will include "a wide range of test and experimentation objectives."
"These operational demonstrations and experiments comprise next-generation technologies including laser communications and the highest performing quantum inertial sensor ever tested in space," the service said in a statement last month.
"Mission 8 will contribute to improving the resilience, efficiency and security of U.S. space based communications architectures," it added.
About the size of a small bus, the X-37B US space drone looks like a mini version of the manned space shuttles retired in 2011.
On previous missions, the X-37B has carried out tests for US space agency NASA.
In operation since 2010, the X-37B Orbital Test Vehicle was designed for the Air Force by United Launch Alliance by Boeing.
It is 30 feet (nine meters) long, has a 15-foot wingspan and is powered by solar panels.
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