Merino's late strike takes Spain past Portugal into World Cup quarter-finals
The Arsenal midfielder collected a pass from Ferran Torres inside the Portugal penalty area and slotted the ball past the Portugal goalkeeper Diogo Costa.
He ran to the cornerflag to celebrate.
Portugal substitute Bernardo Silva and Joao Neves both went close to equalising but Spain, who won the European championship in 2024, held on to set up a last eight date with either Belgium or the United States.
Both teams had shown their attacking intent early, Spain's Mikel Oyarzabal tested Costa from distance and Portugal's Joao Cancelo lashed a shot over the crossbar.
Opportunities for Spain and Portugal
Oyarzabal should have put the Spaniards ahead after eight minutes when a slick move sliced through the Portuguese midfield with Dani Olmo's first-time pass releasing Oyarzabal who dragged his shot wide of Costa's left post.
Bruno Fernandes then dispossessed Pedri to set up Ronaldo but the Spain goalkeeper Unai Simon blocked the effort.
Costa was then called on to make two saves in quick succession, denying Lamine Yamal from the right and, more spectacularly, Alex Baena's follow-up with a full-stretch, fingertip effort.
Simon blocked Joao Felix's header before pushing away Ronaldo's volleyed effort on the rebound, while Nuno Mendes went closest of all with a shot that was redirected onto the bar by a deflection off Pedro Porro as Portugal finished the half on top.
'High standard players'
But it was Merino, an 85th-minute replacement for Olmo, who made the breakthrough to vindicate the gamble of Spain boss Luis de la Fuente.
"We wanted to make sure we pushed forward as far as we possibly could," said de la Fuente.
"We thought the substitutes might give us the spark we would need in extra time," added the 65-year-old.
"Having only just come on, the truth is that the contribution of those players was brilliant. That's our good fortune ... we've got 26 players of the very highest standard."
De la Fuente's opposite number, Roberto Martinez, is likely to come under fire for continuing with Ronaldo in attack while €75m worth of talent in the shape of Gonçalo Ramos remained on the substitutes' bench. Portugal skipper Cristiano Ronaldo said before the game aganst Spain that the 2026 World Cup would be his last.
The 25-year-old scored Portugal's winner during the last 32 game against Croatia but stayed out of the fray as his side laboured to crack open the Spanish rearguard.
Ronaldo, 41, said before the last 32 game that the 2026 World Cup would be his last.
He made his first apperance in 2006 in Germany and steps away from the competition two decades later without winning the crown during his six appearances with his national team.