Ronaldo, 18, has made seven appearances for Leeds since signing for the Championship side in September 2015.
Meanwhile, brother Romario has signed for United's development squad on a one-year contract.
"It feels great to have managed this within less than a year of first coming to the club," Ronaldo said.
"Me and my brother have played together for most of our lives - I'm happy to be sharing the same club with him again and I'm happy he's been given the opportunity."
Leeds midfielder Ronaldo Vieira has signed a new three-year deal to stay with the club until 2019 and has been joined by twin brother Romario.