The boy was initially arrested, but the arrest was later lifted because he is under 15 years old.
He is being cared for by social services, says Christian Magnusson, acting police chief in Helsingborg.
It is unclear how the boy feels about the criminal suspicion.
"It's nothing I can comment on," says Magnusson.
Isolated incident
He also does not want to comment on whether the girl and the boy had a relationship with each other.
We initially assess it as an isolated incident and at this time there are no more suspects.
The incident occurred in Furutorpsparken in Helsingborg at around 3 p.m. on Monday.
Knife injuries according to witness
Axel Östlund works with playground safety in central Helsingborg and was at Furutorpsparken on Monday. He suddenly heard a woman screaming and then saw the girl who, according to him, had been stabbed.
"She was in shock. It was very clear," says Östlund.
He picked the girl up in his arms and did everything he could to calm the situation.
People on site were panicking, says Axel Östlund, who also called 112.
The girl was taken to Helsingborg Hospital with serious injuries.
It's unimaginable.
The police do not want to comment on whether it was a knife injury. The girl's condition is now stable.
She is in good health, says Christian Magnusson.
What do you think?