"The deal is done," DiBenedetto said at a press conference after completing the formalities along with Paolo Fiorentino, vice president of Unicredit bank.
"There have been complicated points, but I never felt like giving up. I was determined to overcome the obstacles and thanks to the patience of the banks and our partners, we have passed those difficulties.
"I am proud of my Italian origins and for this reason I will do big things.
"I didn't expect so much attention. I knew of the great passion from the Romans and this has been one of the reasons for doing this. I share this passion. I never played, but my father played in the American Soccer League."
Unicredit last year became co-owner of the cash-strapped club and will have a share of the new ownership company.
The deal, which must be approved by financial regulators, makes three-time champions Roma the first Serie A club with a foreign majority owner.
DiBenedetto told the Gazzetta dello Sport newspaper earlier this month that his aim was to make Roma "one of the top teams in the world".
Roma, whose current star players include Francesco Totti and Daniele De Rossi, are currently sixth in the Serie A, 15 points behind leaders AC Milan.
DiBenedetto told the newspaper he wanted to put the club on financially sound footing and also wanted "a club capable of winning the scudetto (Italian league title) every year and to be finally competitive in the Champions League".
The businessman is no stranger to European football as a partner of New England Sports Ventures, who own the Boston Red Sox and last year bought English team Liverpool.
In Italy there has been speculation over what changes will emanate from the change of ownership to foreign investors with press reports saying that under-fire Chelsea manager Carlo Ancelotti could be brought in at the end of the current season.
Potential player recruits being mentioned are Argentine pair Javier Mascherano of Barcelona and Diego Pastore of Palermo as well as young Lens defender Raphael Varanne to replace the injured Philippe Mexes.