Riera, 19, has graduated from the academy at Barcelona and skippered the side which won the UEFA Youth Champions League title last term.
However, he was subsequently released by Barca and has agreed a two-year deal with Forest, who will be aiming to improve on last season's 11th-place finish under new manager Stuart Pearce.
Prior to the arrival of Riera, Forest had already signed defender Louis Laing and a pair of forwards in the shape of Matty Fryatt and Lars Veldwijk.
Barca released him first, so does that mean he doesn't have what it takes to make it in the big time? who knows, hopefully at Forest they've spotted something they like with him and can help him develop more.- dannyjohnson