Arsenal have completed the signing of Germany defender Per Mertesacker from Werder Bremen.
The 26-year-old is believed to have cost about £8m and has signed a long-term deal at the London club.
"I had a great time at Werder, and owe them a lot, but the move to London is a wish come true," said Mertesacker.
"The English Premier League has for me always represented an interesting challenge."
Manager Arsene Wenger told the club 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."
Arsenal also signed Santos Andre Santos from Fenerbahce.
The defensive reinforcements come off the back of their 8-2 hammering by Manchester United in the Premier League on Sunday.
Centre-back Thomas Vermaelen was injured for the trip to Old Trafford and missed most of last season with an Achilles tendon injury.
Mertesacker was in the last year of his contract at Bremen and the club are not in the Champions League or Europa League this season.
He was part of the German team which beat England 4-1 at the 2010 World Cup in South Africa last summer and was recalled to the squad to face Austria in their European Championship qualifier on Friday after a heel injury suffered at the end of last season.