The country's fifth-biggest utility - which is preparing for a public stock offering next year - succeeds energy company E.ON, which recently announced its sponsorship of the FA Cup.
No financial details have been given.
"The introduction of a new corporate identity is another step on our way to the stock exchange," RAG Chief Executive Werner Mueller said.
Dortmund sought creditor protection following a record annual loss of euros 68million in the year through June 2004.
However, like many top German clubs it boasts strong attendances, with an average crowd of more than 73,500 at the 83,000-seat Westfalenstadion last season.
