The man and the minor are said to have arranged a meeting on social media. The case came to the police at 09.35 on Sunday morning.
The area, a green area by a bathing area in Hässelby, where the rape is suspected to have taken place has been cordoned off during the day. The police have carried out a technical investigation at the scene, including with the help of dogs.
There are currently no arrests. A preliminary investigation into child rape has been initiated.