Torres, complimented by coach Vicente del Bosque afterwards, gave Spain a 11th minute lead although South Korea then gave them a fright when Kim Do-Heong - who plays for National Police Agency - rifled an equaliser a minute before half-time.
A Xabi Alonso penalty and a Santi Cazorla free-kick in a four-minute spell early in the second half put Spain back in control and Alvaro Negredo, who like Torres just make the cut, added a fourth.
Torres' place at Euro 2012 next month looked to be in danger when his erratic form for Chelsea and Spain led him to be dropped from the squad for a friendly against Venezuela earlier this year.
Given another chance, Torres looked sharp from the outset, deflecting Santi Cazorla's shot just wide after two minutes.
"He is one of those players who brings us speed, draws markers away, never stands still, and gets into positions to receive passes behind the back line," said Del Bosque, who is without the injured David Villa for the tournament. "Fernando is very happy to be here, he's euphoric."
Benat was especially impressive and on Wednesday's performance can consider himself unlucky to be among the players who missed the cut for Poland and Ukraine.
Spain quickly regained control after Ignacio Monreal's shot was handled by a defender and Xabi Alonso converted the resulting penalty in the 52nd minute.
Negredo, included in the squad at the expense of Valencia's Roberto Soldado, fired the fourth with an angled shot with 10 minutes left.
