The teenager was assaulted by a man in his 40s at the City Cemetery during the early hours of Saturday 30 May.
Detectives want to speak to a woman who was driving a red car along the Falls Road who stopped for the teenager and gave her a lift to Downpatrick.
The woman was aged in her 30s or 40s, had blonde hair and wore a uniform.
The girl's attacker, who was unknown to her, was of medium build, clean shaven with short black hair.
Det Sgt Gary Chambers appealed for anyone with information about the attack to contact police at the Public Protection Branch on the non-emergency number 101.
Police investigating a serious sexual assault of a teenage girl in a Belfast graveyard are trying to trace a woman who helped the victim after the attack.