Marseille's France Under-21 international midfielder Matthieu Flamini has joined Arsenal.
The 20-year-old played in last season's Uefa Cup final against Valencia, and helped his country's Under-21s to victory in the Toulon tournament.
Manager Arsene Wenger said: "Mathieu may be young, but he already has a good level of top-flight experience.
"He is very powerful, plays with great creativity, and will be a tremendous addition," he told the club website.
Flamini has signed what the Gunners describe as a "long-term deal".
According to French newspaper L'Equipe, there will be no transfer fee as the 20-year-old had yet to sign a professional contract with Marseille.
However, the Gunners will have to pay Marseille compensation for Flamini - and coach Jose Anigo was not happy to lose the midfielder.
"He's a traitor - this is a beautiful treason," said Anigo. "He used me and cheated me."
He added on Marseille's website: "He didn't sign for us only because of the money. It's good, money-wise, that he hasn't signed for us.
"His sporting argument, that he feared he would play less, is no longer valid.
"His argument that he wanted a first-team slot doesn't hold up, unless of course he plans to keep (Patrick) Vieira out of the Arsenal team."
But Flamini hit back at his former manager's accusations.
"This is not a matter of money. I just didn't get on well with the
manager and failed to reach an agreement," he said.
"I realise that Marseille didn't really want to keep me and I was not their
"All I can say is that I have always been very honest with the club who
boosted my career. I just didn't manage to reach an agreement with them and this
is very common in football."
Flamini had played 14 games for Marseille since making his League debut against Toulouse last December.
The signing will enable Arsenal manager Arsene Wenger to strengthen his midfield in the face of increasing speculation over Vieira's future at Highbury.