Real Madrid have announced an agreement to sign highly-rated teenage defender Jesus Vallejo from Real Zaragoza.
Aston Villa have continued their spending spree after securing the services of Rudy Gestede from Blackburn on a five-year contract.
Riera has been with the Catalan giants for nine years and captained the club at all youth levels, but the 19-year-old was told by Barcelona B at the end of last season that he would not be offered a new contract.
The centre-half becomes Forest's fourth summer signing following fellow defender Louis Laing and strikers Matt Fryatt and Lars Veldwijk to the City Ground.
He said via his Twitter account: "Today I start a new chapter in my life at NFFC. I'm really excited about this new project and I'm gonna give all for this team. Thanks so much for the messages I've received."