The German-flagged container vessel Hansa Stavanger was captured about 645km off the southern Somali port of Kismayu on April 4.
The release of the 20,000 tonne ship, owned by Hamburg shipping company Leonhardt & Blumberg, was expected last week but it was delayed after the pirates demanded a higher ransom.
Malaysian ship freed
Somali pirates also freed a Malaysian tugboat and its 11 Indonesian sailors after a ransom was paid off the coast of Somalia, a maritime watchdog said.
The TB Masindra 7 and its attached Indonesian barge ADM1 had been operating under a contract from French oil giant Total when it was seized eight months ago, Kenya-based non-governmental organisation, Ecoterra International, said.
|Somali pirates attacked more than 130 merchant ships last year
The tugboat and barge had been on their way back to Malaysia from Mukallah in Yemen when the vessels were boarded by pirates.
No details about the ransom amount were given.
The eight-month hostage saga is the second longest ship seizure by the Somali criminal gangs.
A Nigerian tugboat and its 11 crew were held for 10 months before their release in June.
Around 200 sailors and at least 12 ships are still being held in the region.
Somali pirates attacked more than 130 merchant ships last year, an increase of more than 200 per cent over 2007, according to the International Maritime Bureau.
Rough seas and international navy patrols have curbed pirate attacks in the Gulf of Aden and Indian Ocean in recent weeks, but experts have warned that the end of the monsoon season could see a fresh flurry of hijackings over the next few weeks.
More than 30 ships from 16 nations, including Nato members and the European Union, are patrolling the waters off the Somali coast to try to ensure safe passage for ships heading to and from the Suez Canal.
On July 12, pirates seized a dhow with an Indian crew of 11 and used it to launch a failed attack on a super-tanker in the Gulf of Aden.
The pirates have previously captured the Saudi-owned supertanker Sirius Star and held it and its cargo of two million barrels of crude oil for two months from November 2008.