The 22-year-old winger completed a medical on Monday and has penned a five-year-deal at St James' Park.
Manager Alan Pardew believes the former France Under-21 international will bring much-needed pace to the side.
The coach told Newcastle’s official website: “Gabriel is a player I've watched for the last couple of seasons since he arrived at Manchester United.
“He has great pace and excellent technique and he has improved his power in the last 12 months and is ready now to make a real impact in the Premier League.
“I contacted Sir Alex about him early at the start of the summer to see if there was an opportunity to bring him to Newcastle, but it wasn't until Wayne Routledge joined Swansea that we were able to bring him in.
"His best years are ahead of him which is exactly what we want at this club, and with the backing of our magnificent fans I'm sure he will prove an excellent signing for us."
Obertan joins Demba Ba, Yohan Cabaye, Sylvain Marveaux and Mehdi Abeid as the latest French-born addition to Pardew’s squad since the transfer window opened.
The French winger will go into Newcastle’s squad for their first Premier League game of the season at home to Arsenal.
The Tyneside club have lost skipper Kevin Nolan to West Ham United, however, and Joey Barton is available on a free transfer.