Former Mallorca and West Brom midfielder Valero, 27, moves a day after Alberto Aquilani joined from Liverpool, Fiorentina said in a statement.
La Viola have now replaced Milan-bound Riccardo Montolivo with two players, although Adem Ljajic is also expected to exit.
Argentine defender Gonzalo, meanwhile, has passed a medical and will sign a contract with Fiorentina.
Fees were not disclosed but both players are understood to have signed long-term contracts.
