She reported being attacked while walking in Chapel Street, Levenshulme, Manchester, on 22 November 2016.
A 16-year-old boy arrested on suspicion of rape faces no further action.
Ch Insp John-Paul Ruffle said: "It is not appropriate for us to pursue a prosecution in this instance due to a number of vulnerabilities."
"The girl is now being fully supported by specially trained officers and partner agencies."
A 12-year-old girl has admitted to police a rape allegation she made "was not genuine" and the case has been dropped.