Police were called to reports of the assault in Watling Park, Burnt Oak at 12.30am on Tuesday.
The Metropolitan Police said officers found a 19-year-old suffering critical injuries.
Detectives have not disclosed the nature of his injuries.
There have been no arrests and the park has been cordoned off as police scour the area.
Barnet Council posted on social media: “We are aware of a serious incident at Watling Park in Edgware and the park is currently closed while the police carry out an investigation.
“We will be working with the police to assist in their investigation and to reopen the park in due course.”
Anyone with information that may help the police investigation is asked to call 101 quoting CAD 174/02Aug. To remain anonymous Crimestoppers anonymously on 0800 555 111.
