Abdi Mohamed, a Somali militia commander, told The Associated Press in a telephone interview that the seamen were released a day after more than $800,000 in ransom was paid. He did not say who paid it.
A South Korean official in Africa said on Saturday that the hostages included eight South Koreans, nine Indonesians, five Vietnamese and three Chinese.
Somali fighters say that the boat, operated by Dongwon Fisheries, was seized while defending their waters from illegal fishing.
South Korea has said that the pirates seized the vessel, which was on its way to the Seychelles, in international waters and then took it to Somalia.
Piracy is an increasing problem in the region, according to the International Maritime Bureau.
Attacks on vessels carrying food aid for Somalis have hindered UN efforts to provide relief to drought victims.