The two-year deal means that the ex Bangor Grammar School pupil will be at Kingspan Stadium until at least the summer of 2017.
A product of the Hughes Insurance Ulster Academy, McCloskey made his debut for the province against Newport Gwent Dragons in February last year.
He has made 12 appearances for Ulster so far and was man-of-the-match against Edinburgh earlier this season. He was also voted Bank of Ireland Ulster Player of the Month for November.
His form last season resulted in the 22 year-old being selected on the Emerging Ireland tour to Romania.
Commenting on his contract, Stuart said:
"Every young rugby player wants to play for the first team and wants to sign a professional contract and I feel very fortunate now to have done both of those things with the province that I am from and that I support.
"We have a fantastic group of players, great facilities and excellent coaches so signing the contract was not a difficult decision. As at every top rugby side, competition for starting places is high, but I am aiming to play as much rugby as possible over the next two seasons and to help Ulster be successful."
Bryn Cunningham, Ulster Rugby's Team Manager added:
"Ever since he made his debut last season, Stuart has looked completely at home in an Ulster shirt. He has all the talents and physical attributes required to make it to the very top - his pace, huge strength and skill have resulted in him causing problems for every defence he has been up against.
"He is only going to get better and I am extremely pleased that this talented young player has committed his future to Ulster Rugby."
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