Greater Manchester Police said the girl was attacked in Crow Croft Park, Levenshulme, Manchester at about 23:20 BST on Thursday.
Officers were called to the scene following reports of "concern for a teenage girl", said a police spokesman.
He added that police were carrying out inquiries at the scene, which has been cordoned off.

Train drivers' union Aslef said passengers were forced to leave the train due to a row over running longer, 12-car trains without a guard.
The union is now balloting drivers over industrial action.
Gatwick Express said it had been talking to Aslef for a year and was "puzzled" by the situation.
Trains have been introduced gradually since February, with 12-car trains due to officially replace 10-car services from Monday.
Gatwick passenger services director Angie Doll said a driver was not prepared to carry passengers on Saturday's first 12-car service.
She said: "Unfortunately the train was cancelled and around 30 passengers were moved to a train on the adjacent platform which left seven minutes later.
"The driver did take the train to Gatwick and back but not with any passengers on board."
The company said letting the train out of Victoria without anyone on board was the best solution.
It said the train would otherwise have blocked the busy platform and caused significant disruption, with no time available to split it or take it elsewhere.
Ms Doll said the union was aware the trains were being introduced and drivers had already operated the trains and been given training.
She said the company was open to further talks.
Simon Weller, from Aslef, said the union had told Gatwick Express that members were not prepared to work 12-car trains.
He said: "The problem is running driver-only trains with passengers.
"The problem we have is with passengers boarding and alighting with only one person on the train.
He said the union wanted Gatwick Express to put guards on the trains and said: "The driver-only operation is a legacy of the shoestring economics of British Rail."

A total of 737 clubs have entered the 2017-18 competition, with 370 of those starting their campaign on the weekend 5-6 August when 185 extra preliminary round ties will be played.
The extra preliminary round is the first of six qualifying rounds before the first round proper on 4 November.
The two Northwich teams will be also face each other in the North West Counties Premier Division next season.
Northwich Victoria were beaten 3-0 by Premier League Sunderland at the Stadium of Light 11 years ago.
There are 18 clubs taking part in the FA Cup for the first time this season, including Whitchurch Alport.
The Shropshire club was formed in 1946 and boasts former Northern Ireland striker Jimmy Quinn among its former players.
Friday's draw took place 41 days after Arsenal beat Chelsea in the 2017 final at Wembley.
All extra preliminary round winners will receive £1,500 in prize money.
Click here for the full FA Cup extra preliminary round draw
A 12-year-old girl has been raped in a late night attack at a park, police have said.
[NEXT_CONCEPT]
A train made a return journey from London Victoria to Gatwick Airport without passengers in a dispute over a new service.
[NEXT_CONCEPT]
Northwich Victoria, who reached the second round two seasons ago and the third round in 2006, will face local rivals 1874 Northwich - a breakaway club formed five years ago - in the extra preliminary round of the FA Cup.