Fabio Coentrao has also left Santiago Bernabeu, the full-back returning to Portugal to join Rio Ave until the end of 2018/2019.
Mayoral, 21, was expected to leave the European champion in pursuit of first-team football after Mariano Diaz was signed back from Lyon in a €33 million ($52.5 million) deal.
Sevilla had been linked with the Spain youth international, who spent 2016/2017 on loan at Wolfsburg, but he has instead sealed a move to Ciutat de Valencia.
Neither club has stated whether there is a purchase option included in the deal.
Mayoral made 14 appearances in LaLiga last season, just three of which were starts, and scored three goals.
Coentrao, meanwhile, has headed back to his homeland after a loan spell with Sporting CP last term.