Rodrigo Moreno's goal early in the second half was enough to see off Betis at Mestalla and clinch a first final of any kind for Valencia in 11 years.
The result completed an impressive turnaround in the tie for Marcelino's side, who trailed 2-0 after less than an hour of the first leg three weeks ago only to snatch a 2-2 draw thanks to Kevin Gameiro's injury-time equaliser.
Their edge on away goals left them in a decent position and they managed the game superbly against a Betis side who sit three points and two places above them in LaLiga but now miss out on the chance to contest a final in their home stadium.
In search of an early breakthrough, Betis threatened through Joaquin's well-struck drive from the edge of the box, before Jese Rodriguez saw a clever shot on the turn parried over by Jaume Domenech.
Aissa Mandi then glanced the resulting corner narrowly wide, as Valencia, apparently content to wait for the chance to counter-attack, began to invite more pressure.
However, 11 minutes into the second half, the home side struck. Betis' defence failed to get organised and Mandi played Gameiro onside, allowing the Belgian to chip the ball across to Rodrigo, who volleyed in.
A deflated Betis came close to a lifeline through Mandi and a fierce Sergio Canales strike, but it is Valencia who will meet treble-chasing Barca, who knocked out Real Madrid on Wednesday.
FULL-TmopaefjdlmIFJEKDFJÑLOZFPOAEJSDAÑLXJGPÓVJDFNP´V`DRIPGHERI`THPFsdvnkoitfjeirjpjfdolnjtopewsfJGJSDVKNPEFAJKSNZVCPAFJKAN— Valencia CF English (@valenciacf_en) February 28, 2019
What does it mean? Valencia to meet Barca in first final for 11 years
Inconsistency has blighted Valencia's campaign, but they now have a chance to win silverware for the first time since they lifted this trophy in 2008 after beating Getafe 3-1 in the final.
They will face Barcelona this time, a team who have won the title four years in a row and reached a record six consecutive finals, but failed to beat Valencia in LaLiga this term.
Marcelino's job may just have become a lot more secure, too.
Carvalho deserved better
William Carvalho was an imperious figure in the midfield battle and can rightly feel he, at least, should have been rewarded with a place in the final.
Mandi gave without taking
Mandi's failure to stay in line with the rest of the defence handed Valencia their route to goal, and he failed to take a decent chance with a header that could have given Betis some hope of a comeback.
Valencia host Athletic Bilbao in LaLiga on Sunday, the same day Getafe head to Seville to take on Real Betis.