Chief executive Ivan Gazidis will leave Arsenal on October 31 to take up a similar post at AC Milan, the Premier League club have announced.
Speculation that Gazidis - who oversaw Unai Emery's appointment to replace Arsene Wenger in May and has also been behind the change in management structure at the Emirates Stadium - will depart for Italy has been rife for some time.
Now the move has been confirmed, with Raul Sanllehi, currently head of football relations at the Emirates Stadium, to become head of football when Gazidis heads to Milan.
Meanwhile Arsenal's chief commercial officer Vinai Venkatesham will take over the role of managing director at the same point.
Gazidis, 54, has worked for the Gunners since 2009 and played a major role in introducing roles such as Sanllehi's, as well as that of head of recruitment - held by Sven Mislintat.
Those appointments wrestled some power away from Wenger, whose 22-year tenure came to a close at the end of last season.
Emery has since taken the reins after a thorough interview process headed up by Gazidis, who will now leave to fulfil a similar role at San Siro.
Chelsea midfielder Tiemoue Bakayoko has joined AC Milan on a season-long loan.
The Frenchman moved to Stamford Bridge from Monaco last summer for a reported £40million, but struggled to make the desired impact in the Premier League.
Chelsea said in a statement: "The 23-year-old now makes the temporary switch to the Serie A side who are managed by Gennaro Gattuso and are competing in the Europa League this season."
Milan said the deal includes an option to make the move permanent.
He will be presented to the media on Friday afternoon.
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AC Milan starting XI: Donnarumma; Calabria, Bonucci, Romagnoli, Rodriguez; Kessie, Biglia, Bonaventura; Suso, Cutrone, Çalhanoğlu
Arsenal have been drawn to face Italian side AC Milan in the Europa League last 16.
The Gunners suffered the embarrassment of losing 2-1 at home to Swedish side Ostersund in the second leg of their first knockout round match on Thursday night, but progressed 4-2 on aggregate.
They will now take on Milan, a team with great continental pedigree. The Rossoneri have won the European Cup and Champions League seven times but have struggled in recent years, and are currently seventh in Serie A. Elsewhere, Borussia Dortmund will face Red Bull Salzburg and Aletico Madrid take on Lokomotiv Moscow.
Lazio meet Dynamo Kiev, RB Leipzig have been drawn against Celtic's conquerors Zenit St Petersburg, CSKA Moscow face Lyon - who will be hosting this year's final - while fellow Ligue 1 side Marseille tackle Athletic Bilbao and Sporting Lisbon play Viktoria Plzen.
All opening legs will take place on March 8, with the ties concluding on March 15.