It was reported this morning that Forest were closing in on the teenager and now he has used Twitter to confirm the switch.
"Today I start a new chapter in my life at @Official_NFFC. I'm really excited about this new project and I'm gonna give all for this team," he wrote.
He moves to the City Ground on a free transfer and has signed a two-year contract.
Riera, 19, captained the Barcelona Under-19 side to the youth Champions League title last season.