Greece international Sokratis becomes the second player the Gunners have signed from Dortmund in the last year after Pierre-Emerick Aubameyang joined for a club record fee in the January transfer window .
The 29-year-old is an experienced centre-back who shares the same agent as compatriot Konstantinos Mavropanos, another player scouted and signed by Arsenal’s new head of recruitment Sven Mislintat at the turn of the year.
Mislintat maintains a close relationship with senior figures at Dortmund, where he spent over a decade as their chief scout, and played an influential part in Arsenal’s January signings of Mavropanos, Aubameyang and Henrikh Mkhitaryan, the latter being another ex-Dortmund player whom the German knows well.
The signing of Sokratis is Unai Emery’s third acquisition since becoming Arsenal head coach, following hot on the heels of Stephan Lichtsteiner's arrival on a free transfer from Juventus and Bernd Leno completing his move from Bayer Leverkusen , though it is understood that preliminary talks had already taken place between Mislintat and the Greek defender’s agent before Emery took the hot seat at Emirates Stadium.
With Laurent Koscielny ruled out through injury until November at the earliest and Per Mertesacker retiring to take up a new role as head of the Arsenal academy, Mislintat is bolstering a Gunners defence which lacks experience and conceded 51 times in the Premier League during Arsene Wenger's final season in charge.
Speaking to the club's official website, Sokratis said: "It’s a beautiful moment and I’m very happy that I will be here for the next years.
"Arsenal is one of the three biggest teams in the Premier League and one very big club. It has a lot of fans and a very big history.
"Like I said before, I’m very happy that for the next years I will be here to help the team."
The defender’s father, Charalambos Papastathopoulos, told Greek radio station 24/7 in early June that his son rejected late interest from Manchester United to join the Gunners.
Arsenal are also closing in on a move for Sampdoria midfielder Lucas Torreira, though they are unlikley to be able to complete the deal for the Uruguay international until after the World Cup.