Valero, 27, becomes the latest Villarreal star to leave as the club cut their wage bill after unexpectedly suffering relegation from Spain’s top flight last season.
“Pending the usual medical and signing of contract, Villarreal and Fiorentina have agreed in principle to the transfer of midfielder Borja Valero, who will play next season at the Italian club,” said a short message posted online on Wednesday.
“The Madrid native will shortly travel to the Tuscan capital to undergo the required tests and complete the transfer.”
A fee of €7m has been reported for the Spanish international who had an inconsistent last season as the Yellow Submarine went from the Champions League to La Segunda in less than twelve months.
A former Real Madrid youth teamer, he also played in England for West Bromwich Albion in 2008-09 and at Real Mallorca. His only senior cap for Spain came in a friendly against the USA in 2011.
Other departures from Villarreal this summer, as club president Fernando Roig and new coach Julio Velazquez wheel and deal, include goalkeeper Diego Lopez to Sevilla, attacker Nilmar to Qatari side Al-Rayyan, centre-forward Marco Ruben to Ukrainian club Dynamo Kiev and Jonathan De Guzman to Swansea.