Mertesacker has signed a long-term contract with the Gunners and joins the club for an undisclosed fee, reportedly around £10million.
Arsenal boss Arsene Wenger told the club’s official website: “We are delighted to welcome Per Mertesacker to the club.
“He is a German international with vast experience and strength and is good on the floor and in the air.
“I believe he will be well suited to the Premier League and a tremendous asset to the team.”
Mertesacker added: “I had a great time at SV Werder Bremen and I’m grateful to the club. Moving to London enables me to fulfil a dream and a further step in my career.
“The Premier League has always represented a great challenge for me.”