The four-goal win completed an 8-3 aggregate victory over third-tier club Mirandes, with Bilbao set to face Barcelona in final.
Bilbao cruised to a 3-0 lead within the first 22 minutes with goals from Iker Muniain, Markel Susaeta and Jon Aurtenetxe and held that lead until the half-time break.
Aitor Blanco got a goal back for the visitors just before the hour, when the defender poked a rebound into the roof of the net after the ball had struck the post.
But any thoughts of a dramatic comeback were cut short when the home side's towering striker Fernando Llorente scored twice in four minutes.
The first, in the 71st minute, was an exquisite chip from the in-form striker after being put through one-on-one with Mirandes' goalkeeper Cecilio Nauzet.
Llorente then put the game beyond doubt, diving in to convert a cross with the outside of his boot.
There was still time for another scrappy Blanco goal, an unlucky own goal by Mirandes' Cesar Caneda, who diverted a cross past his keeper to make the final score 6-2.
The result means that the Basque club still has a chance to win the Copa Del Rey for the first time in 30 years.
Barcelona will contest the Copa del Rey final for the third time in four years after overcoming Valencia 2-0 at the Camp Nou on Wednesday.
With the cushion of Carles Puyol's away goal from the 1-1 draw at the Mestalla, the Catalans were in imperious form as Cesc Fabregas' first-half strike and a Xavi thunderbolt were enough to help Barca to a 3-1 aggregate victory.
