The deal will allow Yahoo! to charge users for premium content on the sites. Fifa and Yahoo! will share revenues generated from the websites. Exact fees for the contract were undisclosed.
The deal makes Yahoo! the tournament's 15th and final sponsor. Although the financial terms of the deal have been kept under wraps, the other sponsorship deals are understood to be worth $20m (£13.6m) each.
The deal was struck by Fifa Marketing, the division formed when Swiss sports marketing firm ISL/ISSM went bankrupt. The collapse of this company had been expected to hinder Fifa's completion of marketing agreements, but the Yahoo! deal meets next year's tournament's target.
Fifa president Sepp Blatter said, "In the three months since we restructured our marketing activities, we have been able to achieve our aim of completing our group of official partners."
Next year's World Cup will be held in Korea and Japan. The website is to be available in six languages. The 2006 World Cup is to be held in Germany.
German media giant Kirch Group acquired the TV rights to the World Cup when ISL/ISMM went bust earlier this year. It is currently embroiled in a battle with ITV and the BBC over the price of the rights to screen the competition in the UK.
Kirch is believed to have overpaid for the rights to the 2002 and 2006 games and is said to be trying to recoup some of the money by charging exorbitant rates to broadcasters.
However, the World Cup is a protected sport under UK broadcasting legislation, which means it can only be transmitted on free-to-air TV. This leaves the bidding up to the BBC and ITV, which have joined forces to bid for the rights. Channel 4 and Channel 5 are not interested in bidding.
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