The atmosphere inside the stadium was electric from the outset. Athletic nearly stunned the hosts within the opening two minutes when Alex Berenguer rose to meet a cross, but his header drifted narrowly wide of the post.
Real Sociedad gradually settled and created their own opportunities. Midfielder Carlos Soler forced a sharp save from goalkeeper Alex Padilla, who produced an impressive one-handed stop to keep the tie alive.
Soler then squandered a major opportunity before half-time, miscuing his effort with only Padilla to beat.
Both sides relied heavily on their attacking leaders, with Inaki Williams spearheading Athletic's forward play while captain Mikel Oyarzabal led the line for Sociedad.
Athletic improved noticeably after the interval after conceding roughly two-thirds of possession in the first half. Williams burst past Sergio Gomez during one dangerous attack, though the defender recovered quickly to block the shot.
Gomez also made a crucial last-ditch intervention to deny Oihan Sancet a clear chance, while Berenguer later fired wide as the visitors pressed for a breakthrough.
The decisive moment arrived four minutes from time. Oyarzabal stepped up to take a penalty despite attempts at gamesmanship from Padilla and calmly converted. The strike secured the 1-0 win and confirmed Real Sociedad's progression to the final, where they will face Atletico Madrid.
"Very proud of what the boys have done, over the past two months, it's pretty amazing," said Sociedad head coach Pellegrino Matarazzo.
"Our first match was on the fourth of January... and we just reached the cup final.
"The football we're playing is effective and we want to continue... we're in the final and we want to win it."
FC Barcelona manager Hansi Flick has been forced to rethink his defensive plans after losing two key players to injury during the club's dramatic Copa del Rey semi-final clash with Atletico Madrid.
Barcelona were eliminated from the Copa del Rey at the semi-final stage despite winning the second leg at Camp Nou, as Atletico Madrid progressed with a 4-3 aggregate victory.
Barcelona coach Hansi Flick has made it clear his players must believe in themselves and "make the impossible possible" as they prepared to face Atletico Madrid in the second leg of their Copa del Rey semi‑final on Tuesday night.
Barcelona were the biggest winner in Saturday's exciting La Liga action in a day where no fixture ended goalless with crucial points collected at both ends of the league table.
