Cucho Hernández opened the scoring at La Cartuja in the ninth minute before Héctor Fort, on loan from Barcelona, equalised in the 41st minute. Léo Pétrot's straight red card opened the game up and Pablo Fornals lashed a 30-metre winner into the top corner in the 68th minute that sent the Betis crowd into celebration.
Spain had secured a fifth Champions League place for La Liga through UEFA's European Performance Spots system — made possible when Rayo Vallecano's goalkeeper Augusto Batalla saved a 94th-minute Strasbourg penalty in the Conference League semi-final to send the Madrid side to the final.
Pellegrini was visibly emotional in his post-match press conference and began by placing the achievement in the context of his six-year tenure in Seville.
"I'm very happy. It's not easy to have six consecutive seasons in European competition and even less to have reached the Champions League two matches before the end of the championship."
He recalled how the target had been set before a single ball was kicked this season.
"We started the first match of the championship against Elche, and at the end of the team talk, we put up in green letters: 'Today we start the journey to the Champions League.' It was an ambition that many did not expect, given the budgets of other clubs, but we always had the demand and the desire to try to achieve it from the beginning."
He praised the squad's collective resilience through a season of injury setbacks and difficult results.
"The great merit this team had was always getting back up when they were knocked down. The defeats against Atlético de Madrid, Sporting de Braga, the draw in the Seville derby — all those stumbles were today widely rewarded with the great celebration we had in our stadium, with our supporters, to whom we owed this joy."
He singled out the losses as part of a learning process rather than a reason to change course.
"When things don't go well at a given moment, there are two paths: conviction in what you're doing, or believing that because of one defeat you have to change everything. You must not think that because you've gone from 'extraordinary' to 'ordinary' in two matches you need to change the system or the players."
He was generous in distributing credit, particularly to Isco, Lo Celso and Amrabat for their availability despite long injury absences.
"Having Isco out all season, Lo Celso for four months and Amrabat for two months gives the squad even more value. I don't like to single out individuals, but Pablo Fornals has had a great season."
He gave each member of the group the highest possible individual rating.
"I give the full squad a ten out of ten — they knew how to show their faces in the difficult moments."
He was honest about the one moment that still stings.
"The moment when I felt most guilty was the defeat against Sporting de Braga. But if we had gone through one more round in the Europa League and then lost in the final, we would not have had this achievement in the league. I am sure we would not be in the Champions League."
On his future at the club, Pellegrini was measured but unambiguous.
"I have a contract signed and I normally like to honour my contracts. We will sit down and talk about what comes next and how it will be faced — it's a big, major challenge. One never knows what can happen later, but there's no problem in that regard."
He closed with a personal sentiment that captured six years in Seville.
"In six years I have received nothing but gestures of affection. Even in the worst moments, when I went out onto the street, I never found a Betis fan who reproached me for anything we were doing. Now we have to start from zero looking at next season, knowing that the Champions League is a huge challenge — one that needs careful planning."
Real Betis last appeared in the competition in 2005-06, when they faced Liverpool, Chelsea and Anderlecht in the group stage.