Police were called to Lupus Street to reports of a stabbing where they discovered a man, aged 30, with stab wounds at 3pm on Friday.
Police said he was taken to the hospital where his condition is described as critical.
Witnesses said the man was conscious as he was being loaded into an ambulance.
A Met spokesman said there have been no arrests but enquiries continue.
Police said there were road closures around Churchill Gardens, including Lupus Street, Glasgow Terrace and Cumberland Street., and advised drivers to avoid the area.
A London Ambulance Service spokesperson said: “We were called at 2:48pm today (March 19) to reports of a stabbing on Lupus Street, Pimlico.
“We dispatched a number of resources including medics in cars, an ambulance crew and an incident response officer. We also dispatched London’s Air Ambulance.
“We treated a man at the scene before taking him to hospital.”
Anyone with information is asked to contact police referencing CAD 4376 of 19 March 2021.