The news of the move was broken on Ullrich's personal, German-language website.
"I am convinced that I can achieve my goals best with T-mobile," said Ullrich.
Ullrich left Team Telekom in 2002 and floundered in the wilderness before being picked up by Team Coast, which was a financial wreck. Ullrich, and Team Coast, was rescued by the team's bike sponsor, Bianchi.
"It does not please me to leave Bianchi," said Ullrich.
"I waited with my refusal up to the last moment, in order to give Bianchi boss Tony Grimaldi and team manager Jacques Hanegraaf the chance to be able to set up a competitive team."
At Interbike Las Vegas, Tony Lo, president of Giant, confirmed that his company is transferring its support of the ONCE team, which is closing down, to the T-Mobile team. And the former Team Telecom will also ride with mostly Shimano components, not Campagnolo.
We are delighted that one of the most successful teams in road cycling has selected Giant to be their weapon of choice for the coming years," said Lo.
T- Mobile team boss Walter Godefroot said:
For some time we have been following Giants significant technical expertise and the continuous innovations they bring to their bikes, and we feel lucky and confident that we have secured this leading bicycle manufacturing as our partner. We feel with their backing we can make another step forward next year.
The Team bike will be built with the latest ten-speed Dura Ace group from Shimano. Giant also plans to make a team replica version available for consumers.