Barca were aiming for a fourth straight clasico win this season and looked set for the final having edged a 1-0 win in the first leg of the semi-final last month.
However, Benzema and Co were all over Barca on Wednesday. Vinicius Junior opened the scoring just before the break as Carlo Ancelotti's side mounted a spectacular comeback, with Benzema adding a second-half treble.
After netting three at the weekend against Real Valladolid in La Liga, Benzema repeated the trick in front of almost 95,000 distraught Barca fans.
"It was a complete game - when you play a complete game, you win 4-0 like that," said Ancelotti.
"This is the most important moment of the season and when we're at the right temperature (like this), we're good."
Tempers flared with players from both sides getting involved in skirmishes but Benzema had the last laugh to complete a magical night for Real and one Barca will want to forget.
Player ratings from the match can be seen in the picture gallery above. To view the next image, click on the arrows or swipe if on a mobile device.