Emergency services were called to Baylis Road, by the entrance of Waterloo Millennium Green, yesterday (October 8) at 7.46pm.
Medics arrived in less than four minutes, and a man was treated for a head injury.
He was then taken to hospital as a priority.
A police cordon has been in place since the attack, with the park still shut off today (October 9).
Police cordon in Waterloo (Image: Ezekiel Bertrand)
Tape is surrounding the entrance on Baylis Road and officers are at the scene.
The Metropolitan Police has been approached for more information.
A London Ambulance Service spokesperson said: “We were called at 7.46pm on Wednesday 8 October to reports of a stabbing on Baylis Road, SE1.
“We sent an ambulance crew and a clinical team manager to the scene.
“Our first paramedic arrived