After months of speculation, the Gunners reportedly agreed to pay over the release clause in order to do the deal in instalments instead of one lump sum.
With Thomas Partey and Jorginho having left the club following the expiration of their contracts, the signing of Zubimendi helps bolster Mikel Arteta's midfield as they look to bridge the gap to Premier League champions Liverpool.
On signing for Arsenal, Zubimendi said: “This is a huge moment in my career. It's the move I was looking for and one I wanted to make. As soon as you set foot here, you realise how big this club and this team are.
“I set my sights on Arsenal because their style of play is a good fit for me. They have shown their potential recently and the best is yet to come.”
The 26-year-old played 236 games for the Spanish outfit, scoring 10 goals and registering nine assists. He had an important role in Sociedad's Copa del Rey triumph in 2020, ending a 34-year trophy drought.
Zubimendi was also part of the Spain squad that won the European Championships in 2024, as well as the Nations League in 2023.
Arsenal's pre-season gets underway on July 23rd with a clash against AC Milan.