The European Union's anti-piracy naval force says the Thai Union 3 was captured Thursday north of the Seychelles islands, the area of many recent pirate attacks.
The ship is believed to have 25 crew members on board. The EU force says the vessel appears to be heading for Somalia.
Meanwhile, pirates say they have transferred a British couple seized earlier this week to another ship and are holding them close to the Somali coast.
Paul and Rachel Chandler, both in their 50s, were sailing their yacht from Seychelles to Tanzania when they were captured last Friday.
Pirates speaking to reporters by telephone say the couple is now being held on another hijacked ship off the coast of Haradheere, a pirate haven in Somalia's Puntland region.
The EU naval force said Thursday that it spotted the Chandlers' yacht, the Lynn Rival, floating abandoned in the ocean.
Somali pirate hijackings have picked up this month after a lull of several months, caused in part by the regional monsoon season.
The pirates are now believed to be holding eight ships overall. Pirates usually release the ships and their crews after payment of ransom, sometimes in the millions of dollars.
