McEwan, 56, had been tipped to take the reins after predecessor Stephen Hester was ousted by the government in June. He will take over in October with the job of completing RBS's restructuring, ensuring its shares rise above the government's break-even price so the stake can be sold.
His promotion comes less than a year after he arrived to run RBS's retail arm.
British Finance Minister George Osborne said McEwan had impressed with his vision of RBS, 81 percent owned by the government after a 46 billion pound ($70 billion) bailout during the 2008 financial crisis, as a "strong, UK-centered corporate bank", focused on supporting the UK economy.
"I think he'll provide the leadership RBS needs as the bank puts the mistakes of the past behind it, and the government seeks to get the best value for the taxpayer from the money the last government put into the bank," Osborne said.
Chairman Philip Hampton, who has overseen the search for Hester's successor, said McEwan had "emerged as the best candidate" and was the only person to be offered the job after the bank had considered internal and external options.
Hampton said he didn't expect McEwan's appointment to mark a change in strategy, pointing out RBS had already refocused as a retail bank under Hester's stewardship, shrinking its investment bank to account for 20 percent of operating profit compared with 60 percent prior to its rescue.