[14] In July 2017, Merino joined Newcastle United on a season-long loan; the Premier League side agreed to a clause obligating them to sign the player permanently, based on a number of appearances.

[18] He scored his only goal with a late header in a 1–0 win over Crystal Palace at St James' Park on 21 October 2017,[19] also providing a sole assist.

[18] Merino returned to Spain on 12 July 2018, signing a five-year deal with Real Sociedad for an undisclosed fee[20] reported to be €12 million.

[21] His first match in La Liga took place on 18 August when he played 59 minutes in a 2–1 win against hosts Villarreal,[22] and his first goal on 21 September won the game at Huesca.

[23] Despite struggling initially to adapt to his new team, also dealing with some injury problems, he eventually became a starter,[24][25] scoring four times in 32 fixtures in his first season.

[29] On 3 April 2021, Merino won his first title after playing the full 90 minutes of the 2020 Copa del Rey final – delayed due to the COVID-19 pandemic – against Athletic Bilbao, assisting Portu who was brought down in the box at the hour mark; Mikel Oyarzabal converted from the spot for the only goal at the Estadio de La Cartuja, and Merino was named player of the match.

[41] On 20 August 2020, Merino received his first call up to the senior side, for the initial two matches of the 2020–21 UEFA Nations League against Germany and Ukraine.

[45] In the quarter-final against Germany in Stuttgart, he scored a header from a cross by Dani Olmo in the 119th minute to secure his nation a spot in the semi-finals with a 2–1 win;[46] his celebration after the goal (running around the corner flag) mirrored that of his father after he had done the same for Osasuna in a UEFA Cup fixture at the same stadium in 1991.

[47] He also featured in Spain's win in the final where they defeated England 2–1, achieving a continental record of four European championships,[48] and in the process became one of 12 players who won both youth and senior Euro titles, alongside his teammates Nacho, Ferran Torres, Álvaro Morata and Rodri.