Sevilla have exercised their option to make Suso's loan from Milan a permanent transfer.
Suso, who arrived at the Ramon Sanchez Pizjuan on an 18-month loan in January, has signed a five-year deal and cost Sevilla a reported €21million.
He made 19 appearances in all competitions, scoring once and setting up another as Julen Lopetegui's side secured Champions League qualification by finishing fourth in LaLiga.
It was confirmed last week that Simon Kjaer had gone in the opposite direction, having been on loan at Milan since January.
While their LaLiga campaign is over, Sevilla face Roma in a Europa League last-16 match in Duisburg on August 6.