Eggert Magnusson has stepped down as chairman of West Ham and sold his stake in the Premier League club.
Magnusson says it was an "honour" to have been part of the club
Magnusson, who was part of the Icelandic consortium that bought the Hammers a year ago, had already quit as executive chairman in September.
"Now that I need to focus a little more on my interests overseas, I think it is the right time to stand down," he said.
Owner Bjorgolfur Gudmundsson has become chairman and executive chairman and has bought Magnusson's 5% shareholding.
"Eggert Magnusson has done a tremendous job as chairman, often in difficult circumstances, since our consortium bought the club 12 months ago," said Gudmundsson, who the club says has invested a further £30.5m.
"Eggert has decided to move on to new pastures and to pursue his other interests. We wish him well for the future and express our thanks for all that he has done for West Ham."
Magnusson has had an eventful time with the Hammers over the last 12 months.
Within weeks of taking over, manager Alan Pardew was sacked and, with Alan Curbishley at the helm, the club survived relegation by three points.
Magnusson also had to deal with the long-running Carlos Tevez saga, which drew to a close when the Argentine striker completed his move to Manchester United after months of legal rows.
However, Magnusson insisted: "I have thoroughly enjoyed my time here at West Ham. From the moment that Bjorgolfur Gudmundsson and I formed the consortium to bid for the club, it has been an honour to be involved.
I am particularly delighted that we have been able to take the first real step in our plans for a new stadium
Hammers owner Bjorgolfur Gudmundsson
"The last 12 months have not been without their challenges but I feel that we have emerged stronger and fitter as a club.
"I would also like to pay tribute to the West Ham fans who, I believe, have shown themselves to be the best in the country.
"My thanks to everybody who has worked with me in my time here and I wish the club every success for the future."
Gudmundsson has expressed his determination to push ahead with a move to a new £250m, 60,000-seater stadium, with a site having been identified at an old Parcelforce depot next to West Ham tube station.
"We are seeking to build some truly solid foundations for the future ambitions of West Ham," he added. "This is a great football club, with tremendous heritage and tradition and the most wonderful set of supporters.
"My job as owner is to help plan for the future and create the right conditions off the field to help Alan Curbishley and the playing squad deliver success on the field.
"I am particularly delighted that we have been able to take the first real step in our plans for a new stadium, which is crucial to the long-term ambitions of the club."