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.
It was not all plain sailing for the world and European champions as South Korea, themselves warming up for a pair of World Cup qualifiers, exposed some gaps in the Spanish rearguard in the first half.
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.
But his Champions League semi-final goal for Chelsea against Barcelona, when he broke from his own half to score the goal that made the tie safe for the Premier League side, helped swing the pendulum back in his favour.
Given another chance, Torres looked sharp from the outset, deflecting Santi Cazorla's shot just wide after two minutes.
The 28-year-old forward put Spain ahead when he got behind the South Korea defence to meet a long Benat cross into the area with a backwards header which flew into the net.
It was his first goal since he hit the target in a 4-0 friendly win over the United States on June 4 last year and the 28th of his international career.
"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."
Spain, still without their contingent of Barcelona and Athletic Bilbao players after they were involved in the King's Cup final last Friday, completely dominated possession for the next 15 minutes, barely letting South Korea out of their half.
HELPLESS REINA
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.
After the half-hour mark, the Koreans began finding gaps in the Spanish defence and, having already caused a couple of scares, levelled just before the break.
Kim was left free on the edge of the area and rifled a 25-metre shot past the helpless Pepe Reina.
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.
Four minutes later, midfielder Santi Cazorla scored with a low free-kick which went under the Korean wall.
Negredo, included in the squad at the expense of Valencia's Roberto Soldado, fired the fourth with an angled shot with 10 minutes left.
Spain,
