The country has been without a functioning central government since 1991, allowing pirates to operate almost uninhibited in one of the world's busiest shipping lanes.
In this season it is hard to take ships because monsoon winds make the seas rough. No-one expected attacks at this time
Omer Ozgur, from Horizon Shipping, said the Horizon I was continuing on its course despite the hijack.
The pirates have not yet issued any demands or contacted the firm.
Andrew Mwangura, of the East African Seafarers' Assistance Programme, which works to free ships, said the attack came as a surprise.
"In this season it is hard to take ships because monsoon winds make the seas rough. No-one expected attacks at this time," he told Reuters news agency.
Earlier, Nato spokesman Commander Chris Davies told the BBC's Network Africa programme that pirates in the Gulf of Aden were having less success this year compared with last year.
But he said Nato, which has an anti-piracy task force off the Horn of Africa, wanted the legal apparatus in place in Africa to deal with the pirates if they were caught.
"If we capture the pirates we're not looking to take them all the way back to, say, America or Turkey," he said.
Earlier in June the EU, which co-operates with Nato in the region, agreed to extend its anti-piracy operation there until the end of 2010.
Two dozen ships from European Union nations, including Britain, France, Germany and Italy, patrol an area of about two million square miles.
When first loaded, the map's focus falls on Somalia where most of the pirates are based. Use the arrow icons to scroll left towards Europe and the United States which are both playing a central role in tackling the problem.
Scroll to the right for a story about the Philippines, which supplies many of the world's mariners.
You can zoom in for more detail by using the "+" or "-" signs on the upper left hand side.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.
Bookmark with:
What are these?