Osasuna Born in
Pamplona,
Navarre, He made his senior debut with
the reserves in the
2013–14 season, in the
Tercera División. He scored his first professional goal on 21 December, the winner in a 2–1 away victory over
Las Palmas. Merino was promoted to the main squad on 31 January 2015, being given the number 8 jersey. He was a regular starter in his
first year, helping them to narrowly avoid relegation. In the
2015–16 campaign, Merino scored four goals from 34 appearances as Osasuna finished sixth and reached the
promotion play-offs. In that stage's first round, he netted a brace in a 3–1 home win against
Gimnàstic de Tarragona, and added another goal in the second leg (3–2 victory).
Borussia Dortmund On 15 February 2016, Merino signed a five-year deal with
Bundesliga club
Borussia Dortmund, which was made effective on 1 July. His first league appearance occurred on 14 October, when he played the full 90 minutes in a 1–1 home draw against
Hertha BSC.
Newcastle United 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. On 13 October, this was invoked, and he agreed to a five-year contract. Merino made 25 competitive appearances for the Magpies. 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, also providing a sole
assist. reported to be €12 million. His first match in
La Liga took place on 18 August when he played 59 minutes in a 2–1 win against hosts
Villarreal, and his first goal on 21 September won the game at
Huesca. Despite struggling initially to adapt to his new team, also dealing with some injury problems, he eventually became a starter, scoring four times in 32 fixtures in his
first season. On 7 March 2020, Merino wore the
captain's armband for the first time in a 1–0 league defeat away to
Barcelona. In July, he renewed his contract until 2025. 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. In the
2023–24 campaign, Merino won the most duels for possession in Europe's top leagues at 326.
Arsenal Merino returned to the English top division on 27 August 2024, signing a four-year deal at
Arsenal for a reported fee of £31.6m. In his first training session, he fractured his shoulder in a collision with
Gabriel Magalhães and was ruled out for several weeks. He featured in his first match on 1 October, as a
substitute in the 2–0 home win against
Paris Saint-Germain in the
league phase of the
UEFA Champions League. He scored his first league goal 26 days later, heading home from a
free kick in a 2–2 home draw with
Liverpool. On 15 February 2025, Merino replaced
Raheem Sterling in the 69th minute of the league fixture at
Leicester City; as the habitual starter at
centre-forward,
Kai Havertz, had suffered a hamstring injury in training a week earlier, he was slotted into Havertz's position, going on to score both goals in the 2–0 victory. On 8 April, having again featured as a striker, he closed the 3–0 defeat of
Real Madrid at
Emirates Stadium in the Champions League quarter-finals' first leg. In his
first season, he was regularly deployed in both positions. On 28 September 2025, Merino scored his first goal of the
new campaign in a 2–1 away win over his former employers Newcastle, coming off the bench to spark a late comeback in the final ten minutes. With summer signing
Viktor Gyökeres out with a hamstring injury, he assumed his makeshift striker role once again in late November matches against
Sunderland,
Tottenham Hotspur,
Bayern Munich and
Chelsea. ==International career==