Jack Cork wants to open new contract talks with Southampton, but remains focused on establishing his first-team place.
Everything is in place for Cristiano Ronaldo to make a dream return to Manchester United, says Guillem Balague.
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."