Obertan, whose first team chances with United were limited during his two years at Old Trafford, is relishing the prospect of regular football with Newcastle.
"I have come here with a lot of ambition," he told Sky Sports News. "I just want to play football."
Newcastle boss Alan Pardew was delighted to have landed a long-term transfer target.
"It's good news for us. He is a player we have been after for some time,' he said.
"With Wayne Routledge moving on there was an opportunity to bring him in.
"With his pace, power and versatility he is going to be an important player for us."
The 22-year-old Frenchman moved to United from Bordeaux in the summer of 2009 for £3 million (Dh18.03m).