A Japanese cargo ship was seized by pirates off the coast of Somalia late on Saturday, South Korea's Yonhap news agency quoted Seoul's foreign ministry as saying on Sunday.
The cargo ship was carrying 23 sailors, including five South Koreans, Yonhap said.
Details on the pirates and the safety of the sailors were not immediately available, it said.
Heavily armed Somali gunmen have seized more than 30 vessels so far this year, making the busy shipping lanes through the Gulf of Aden the most dangerous in the world.
South Korea is considering dispatching a naval ship to the area to protect its vessels and sailors, Reuters reported.