Bale was hailed as the "Prince of Wales" in the Spanish media on Thursday after he collected the ball wide on the left near the halfway line in the 85th minute at Valencia's Mestalla stadium, shrugged off the attentions of FC Barcelona centre back Marc Bartra and charged towards goal.
Arriving in the penalty area and with Barca defenders bearing down on him, he coolly slotted the ball between the legs of goalkeeper Jose Manuel Pinto to secure a 2-1 success and set off wild celebrations in the Spanish capital.
It was a breathtaking end to the 228th meeting between the arch rivals and Bale proved he can step up on the big occasions after a debut season disrupted by niggling injuries, during which many questioned his exorbitant price tag.
His performance also helped Real make light of the absence of injured top-scorer Cristiano Ronaldo and Bale easily outshone FC Barcelona's four-times World Player of the Year Lionel Messi, who had another quiet game, and their marquee signing of the close season Neymar.
"It is certainly the most important goal of Bale's career," Real coach Carlo Ancelotti told a news conference. "It came at a decisive point of the match."
FOUNTAIN OF JOY
Before joining Real from Tottenham Hotspur for a fee of 100 million euros ($138 million) in the close season, the only trophy in Bale's cabinet was an English League Cup won with Spurs in 2008.
Thanks to his stunning effort, Real are still in with a chance of becoming only the second team after FC Barcelona in 2008-09 to win a treble of La Liga, Champions League and King's Cup.
They host holders Bayern Munich in their semi-final, first leg in Europe's elite club competition on Wednesday and are three points behind leaders Atletico Madrid in La Liga with five games remaining.
Bale told reporters scoring the winner in the Cup final was "a dream come true".
"Not just because I scored but because of the work of the whole team," the 24-year-old added.
"It was really special. We've won the first of the three available titles and we have to keep fighting for everything.
"This win is for all the fans and we hope that it will be the first of many to come."
After collecting the trophy, Bale and his team mates flew straight back to the capital to join some 70,000 Real fans at the traditional celebrations around the central Cibeles fountain.
If he can produce more goals of the quality of Wednesday's effort in Real's remaining games this term, Cibeles could soon be hosting another party or two and the club will consider 100 million euros as money well spent.
Soccer-La Liga fixtures
April 18 (Infostrada Sports) - Fixtures from the La Liga matches on Friday
Friday, April 18 (GMT)
Atletico Madrid v Elche (1830)
Saturday, April 19 (GMT)
Osasuna v Valencia (1400)
Levante v Getafe (1600)
Real Sociedad v Espanyol (1800)
Sunday, April 20 (GMT)
Almeria v Celta Vigo (1000)
Rayo Vallecano v Real Betis (1500)
Sevilla v Granada CF (1700)
Barcelona v Athletic Club (1900)
Monday, April 21 (GMT)
Malaga v Villarreal (2000)