WTO Director-General Pascal Lamy, Swiss federal minister for the economy Johann Schneider-Ammann, and Charles Beer, head of the Geneva cantonal government were among the officials at the site for the ribbon cutting.
The 130-million-franc project adds 20,000 square metres of floor space to the headquarters, allowing the organization to regroup all its operations under one roof.
Since the WTO 15 years ago took over the Centre William Rappard, a building originally built in 1926 to house the International Labour Organization, its staff has been scattered in several buildings.
The new addition, at the site overlooking Lake Geneva, and a modernization of the existing building, was financed by the Swiss government with a grant of 70 million francs and an interest-free loan of 60 million francs.
It was built despite several obstacles, including a campaign launched to stop the project because of concerns that it impinged on public park space.
Voters in 2009 eventually approved the expansion in a city referendum.
Despite such holdups, Schneider-Ammann noted in a speech at the opening ceremonies that the project was built on time and on budget, the ATS news agency reported.
The WTO flung open its doors to the public on Sunday, offering guided visits and art expositions, with a restaurant and bar open all day.
Founded 15 years ago out of the ashes of the General Agreement on Tariffs and Trade (GATT), the WTO is the only international body that deals with the rules of trade between nations.
The WTO secretariat employs 629 regular staff in Geneva, including individuals from 70 nations, according to the organization’s website.