Yamaha Tech 3's Dovizioso, 26, will join team-mate Nicky Hayden at Ducati until 2014 after seven-time MotoGP world champion Rossi opted to switch back to Yamaha next season.
"The agreement reached with Andrea Dovizioso confirms the company's primary interest in racing, which is an active and strategic part of Ducati's DNA and heritage," said President Gabriele Del Torchio in a statement.
"It is with great pleasure that I welcome Andrea, as we marshal our best efforts in confronting the MotoGP World Championship."
As preparations begin for this weekend's Czech Republic Grand Prix, Ducati also revealed Hayden will remain off-track after the American injured his right hand during a qualifying crash at Indianapolis last weekend.
Hayden will not be replaced for the race meaning Ducati's only representative will be Rossi.
"It's tough to miss another race. I don't have big problems, but my hand is still very swollen and I don't have much strength, and of course right-hand injuries are worse for motorcycle racers," Hayden said in a statement.
It is expected Hayden will return to action at the San Marino Grand Prix in Misano on Sept. 16.
Spanish rider Jorge Lorenzo leads the MotoGP championship by 18 points ahead of countryman Dani Pedrosa and Australian Casey Stoner, who retires at the end of the year.