The defendant, identified only by the surname Ahn, was convicted of plotting to kill Park Sang-hak -- who also had fled North Korea -- with a poisoned needle, Yonhap News Agency reported.
Ahn was fined 11.75 million won ($10,399), the South Korean news agency said.
In its ruling, the Seoul Central District Court said it took into consideration "the fact that he was unexpectedly given the poisoned needle while gathering North Korea intelligence for the National Intelligence Service," the South's spy agency.
Ahn defected to South Korea in 1995 and was employed by a company handling inter-Korean economic projects. He met a North Korean spy in 2010 during business trips to Mongolia, and was later ordered by the North to carry out the assassination, court officials said.
Ahn said he tried to carry out the order because the South's intelligence agency had rejected his offer to gather intelligence on the communist regime after alerting the agency to the assassination plot.