The teenager, who had denied the charge, was one of a gang who lured their 21-year-old victim to High Hazels Park on November 23, police said.
A second boy, 16, was also sentenced to two years for sexual assault following the two-week trial at Sheffield Crown Court.
Neither of the boys can be named for legal reasons.
South Yorkshire Police said its initial investigation led to the arrest of four teenagers, one of whom was released with no further action.
The third, a 15-year-old boy, was charged with attempted sexual assault and found not guilty at a hearing in August.
Temporary Det Ch Insp Ben Wood said: “This was an incredibly distressing incident in a very public place, and I would like the commend the victim for her bravery.
“There is no doubt that her ordeal with have a lasting impact on her, and no sentence or court result can undo the damage that these individuals have caused.”