The 17 year-old midfielder recently made his first team debut for River, but has been pursued by Barca since he was 12 years of age.
"I have recently been tracked by Barcelona, but I intend to continue my career with River," revealed Lamela. "I still have much to learn.
"Today I am pleased to be part of this club. People are starting to know me.
"River Plate is currently experiencing a difficult period, but I am sure we will cope."