
The 21-year-old France international has signed for an undisclosed fee believed to be around 12 million pounds (24 million dollars).
"I am very happy to become an Arsenal player, it means such a lot to me," said Nasri.
"This is a very important step in my life as a football player because Arsenal is a massive club.
"I really hope everything will go well for me here. I will do my best for Arsenal and its fans, and I am honoured to be part of such a great football club."
Manager Arsene Wenger was delighted finally to have got his man.
"We are delighted to be signing Samir," he said. "He is young, quick and technically an outstanding player.
"He has shown great potential with his performances for Marseille and the French national team over the past two seasons, and he will add great quality to our squad."
DPA