Andrew Mwangura, coordinator of the East African Seafarers' Assistance Program, said the vessel was Horizon-1.
"It was taken in the Gulf of Aden this morning with 23 Turkish crew," he said. "In this season, it is hard to take ships because monsoon winds make the seas rough. No one expected attacks at this time."
Mwangura said he believed the ship was carrying sulphate.
Poor weather has hampered pirate attacks of late giving the nearly 20,000 ships that pass through the Gulf of Aden each year some temporary reprieve.
Foreign navies have been deployed off Somalia since the turn of the year to try to prevent attacks but find themselves overstretched given the vast expanses of water involved.