The 300,000-tonne Samho Dream and its 24 crew -- five South Koreans and 19 Filipinos -- were released only after a ransom reported as a record $9 million was given to the pirates in a helicopter air drop.
Piracy has surged in the waters off lawless Somalia in recent years, and international warships patrol the area in an effort to clamp down on the problem.
After the latest hijacking, pirates now hold 29 vessels and 693 hostages off the coast of Somalia.
The chemical tanker was carrying eight South Koreans, two Indonesians, and 11 crew from Myanmar when it was seized in the waters between Oman and India, officials said.
The ship is owned by a Norwegian company and operated by South Korea's Samho Shipping, based in the southern port city of Busan, according to the European Union's anti-piracy mission.
It was 350 nautical miles southeast of the port of Muscat, Oman when it was taken, the EU NAVFOR mission said in a statement, adding that no further details about the attack were avai