The militants known as the Emancipation of the Niger Delta (MEND) said they had released the South Koreans at the request of Mujahid Asari Dokubo, a jailed separatist leader whose freedom the MEND was demanding.
"In fulfillment of our earlier pledge, all five Korean prisoners captured by our unit in the attack on the Daewoo camp were released at 1600 hours today," MEND said in a statement.
The workers, staff of Korean firms Daewoo and Korea Gas, were snatched from a gas plant in southern Nigeria on Wednesday morning in response to a court decision to deny bail to Niger Delta guerrilla leader Asari.
"They are already at government house (in Rivers State)... They are now having lunch. They are in good shape, if not they wouldn't be eating," said provincial government official Magnus Abe.
MEND had said earlier it would release the hostages in exchange for the freeing of Asari, head of the outlawed Niger Delta People's Volunteer Force (NDPVF), who is standing trial for treason in Abuja.
It said the Koreans had been handed over to Nigerian Senator David Brigidi, who confirmed that he had received the men "on the road betweem Port Harcourt and Bukuma" and was taking them to government house in Port Harcourt.
A spokesman for Anglo-Dutch oil giant Shell, at whose gas plant the South Koreans had been working, also confirmed their release.
However, MEND warned that more attacks would follow despite the quick release of the Koreans, and told foreign oil firms to leave the region.
MEND guerrillas armed with rocket launchers approached the gas plant near Port Harcourt by boat early Wednesday and overpowered Nigerian soldiers and policemen who were guarding the site.
Nigerian army and police authorities said at least three security force members died in the attack and four others were injured. Shell was forced to shut down the plant where a number of companies had subcontract operations.
The kidnapping is the latest in a string of similar incidents targeting foreign oil workers in the Niger Delta, a hotbed of unrest by armed separatists demanding a larger share in oil revenues and compensation for environmental destruction due to oil exploration.