Marubeni now holds 55% of the company, after buying a further 5% from Innovation Network Corporation of Japan (INCJ). Financial terms of the deal were not released.
The two Japanese companies each bought 50% stakes in Seajacks in 2012; INCJ retains its 45% holding.
In 2013 a Japanese offshoot of Seajacks was set up by the companies to make the most of Asian offshore wind growth.
Marubeni has been investing heavily in wind power in recent years. Most recently it bought a 25% stake in Irish developer Mainstream Renewable Power for EUR 100 million.
In 2011 it bought a 49.9% stake in Dong Energy's Gunfleet Sands offshore wind farm, located off England's Essex coast.
It has also been involved in plans to build a floating offshore wind farm in Fukushima, Japan.
Seajacks largely works on offshore projects in the North Sea, having recently completed construction of Germany's 288MW Meerwind project.