Martinez signed from Ajax Amsterdam has made 177 career club appearances and is a full Argentina international.
Martinez has won numerous honours, including the 2021 Copa America with his country and two Eredivisie titles with Ajax
Lisandro Martinez said: “It’s an honour to join this great football club. I’ve worked so hard to get to this moment and, now that I’m here, I’m going to push myself even further.
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“I’ve been lucky enough to be part of successful teams in my career and that’s what I want to continue at Manchester United. There will be a lot of work to get to that moment, but I firmly believe that, under this manager and coaches, and together with my new team-mates, we can do it.
“I want to thank Ajax and their fans for all the support they gave me. I had an unbelievable time there but feel the moment is right to test myself in another environment. Now I am at the perfect club to do this.”
John Murtough, Manchester United football director, said: “Lisandro is an outstanding player who will bring further quality and experience to Erik [Ten Hag]’s squad.
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“We are delighted that he has chosen to join Manchester United and we are looking forward to seeing him develop further and help the team achieve the success we are aiming for.”