Police were called to Waterloo Millennium Green, near the entrance at Baylis Road, yesterday (October 8) at 7.40pm.
The man, 25, was treated by the London Ambulance Service as the first set of medics arrived in less than four minutes.
He was taken to hospital, and his condition is believed to be life-changing.
A crime scene currently remains at the park, over 23 hours since the attack.
Tape is surrounding the entrance on Baylis Road and officers are at the scene.
No arrests have been made at this stage.
A Metropolitan Police spokesperson said: “Police were called at 19:40hrs on Wednesday, 8 October to reports of a stabbing on Waterloo Millennium Green, SE1.
“Officers attended and a man, aged 25, was being treated by the London Ambulance Service. He was taken to hospital and his condition is believed to be life-changing.
“A crime scene was put in place and enquiries are ongoing.
“There have been no arrests at this stage.”
Anyone with information is asked to contact police on 101, or online, quoting CAD 7429/08OCT. Alternatively, you can contact Crimestoppers anonymously on 0800 555 111.
