The Radar 5 unit was carried into space on Japan's mainstay H-2A rocket from a launch site in the country's southwest.
It is meant to replace an existing satellite that is coming to the end of its mission.
Japan started putting spy satellites into orbit in 2003 after North Korea fired a mid-range ballistic missile over the Japanese mainland and into the western Pacific in 1998.
The threat has steadily accelerated and just last week Pyongyang fired four ballistic missiles, with three landing provocatively close to Japan.
Tokyo currently maintains three optical satellites for daytime surveillance and three radar satellites for nighttime monitoring. Two of those are backups.
The new satellite will succeed one of the three radar satellites that was launched in 2011.
The satellites are officially for "information-gathering" -- a euphemism for spying -- but are also used to monitor damage in the wake of natural disasters.
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