Police say the woman was walking along an alleyway off Royal Avenue near the Castle Court Shopping Centre in the early hours of Saturday morning.
She was approached from behind by a man who sexually assaulted her.
He is described as black, in his mid-30s, around 5ft 6ins tall and of slim build
The victim has told police that he was wearing navy blue jeans and a checked shirt.
Detectives from the PSNI’s Rape Crime Unit based at Ladas Drive in east Belfast have appealed for anyone who was in the area at the time or saw the woman to ring them on the non-emergency number 101.