Dortmund shares have taken a battering in recent weeks after their defeat by Bayern in the Bundesliga and their poor performance in the Champions League.
"This partnership will help us accept the challenges of the future and make sure Bayern remain a top address in Europe and worldwide," Beckenbauer said of the lucrative deal under which Adidas will also remain Bayern's sponsor until 2010.
Markets did not like the decision by Adidas, the world's second-biggest sportswear makers after Nike, to spend 75 million euros (US$69 million) on the Bayern stake.
Shares fell more than eight percent on the announcement because analysts said the price was too high.
But Adidas said the move would help to stop their rivals from getting a firmer foothold in the German market.
They also pointed to the fact that Bayern are a rich club.
Bayern are expected to report 2001 sales of more than 150 million euros and earnings in the "double-digit" millions, making them the most profitable soccer club in the world, Adidas said.
Despite Adidas' difficulties convincing the markets of the wisdom of their decision, there are signs that soccer clubs are a lot less sceptical.
Reiner Calmund, commercial manager of Bayer Leverkusen, has said his team are considering the sponsorship route.
"Naturally the price has to be right but a partner could take a 10 or perhaps a 20 or 25 percent share," Calmund told Handelsblatt newspaper.
Currently the club is held by Fussball GmbH, a 100 percent-owned subsidiary of chemicals firm Bayer AG.
Among the candidates listed are the utility RWE AG, which is already a major sponsor.
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