The Rev. Han Sang-ryol was previously convicted of visiting North Korea without permission last year, and of praising Pyongyang’s communist system.
The Supreme Court says its decision Thursday to uphold the sentence is final.
North Korea has criticized South Korea’s detainment of Han, saying it demonstrates Seoul’s anti-unification, confrontational policy.
The two Koreas are still technically at war because their war in the 1950s ended with an armistice, not a peace treaty.
South Korea’s National Security Law bans citizens from supporting North Korea’s political system or entering the country without approval.