The 26-year-old arrives from Real Sociedad, for whom he made 236 appearances in all competitions at first-team level after graduating from the club’s academy.
In April 2021, Martin helped Sociedad win the delayed 2019/20 Copa del Rey against intercity rivals Athletic Club, alongside current Gunner Mikel Merino.
Martin 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.”
