The 28-year-old midfielder has signed a long-term deal at the Emirates and will wear the No 23 shirt during his time in north London.
Merino was part of the Spain side that triumphed at Euro 2024, featuring in every game of the tournament as Luis de la Fuente's side defeated England in the final last month.
Speaking after the deal was confirmed, Mikel Arteta hailed his new signings versatility and technical ability.
'Mikel is a player who will bring us huge quality with his experience and versatility,' Arteta told the club's website.
'He has played at the top of the game at both club and international level for a number of seasons now. Mikel will make our squad significantly stronger, with his technical ability, together with his strong and positive character and personality.
'As we all saw in the summer, Mikel is also a winner, with his strong performances throughout the Euros helping Spain win the tournament. We are delighted to welcome Mikel and his family to the club and can’t wait to start working with him.'
In a video shared on the Gunners' social media channels, Merino could be seen introducing himself to his new team-mates.
'Thank you all for the warm welcome,' he said. 'I'm very happy to be here with all of you.
'I'm just excited for this season, hopefully we can keep doing big things as you guys have been doing. Just thank you.'