The girl was attacked between 3.30pm and 4pm on Monday as she was walking through a field in Gorton.
A man, who is described as black and in his early 20s, approached her and pulled her into some bushes before raping her.
Police said the man then ran off and the girl went home and told her family what had happened.
The man is described as about 5ft 6in tall, with short hair. He was wearing a grey tracksuit.
"We are in the early stages of this investigation and we are urging anyone who may have seen the victim being grabbed, or the offender running away, to contact us," said Superintendent Stuart Barton.
"The victim has been left traumatised by what happened but trained officers have been speaking to her and she has fully supported our investigation.
"We understand that this incident will cause some concern in the community but we are treating this as an isolated incident. We also have an increased police presence to provide reassurance.
"While a teenager should never be subjected to such an attack when walking home from school in the afternoon, we would like to remind girls and women of some very basic steps for their safety.
"They should stick to busy roads, walk with others where possible and take public transport if they can."