The boy admitted himself to a south London hospital just before 3pm today (October 10).
His condition is currently unknown.
It is believed that he sustained his injuries a short time earlier in Chislehurst Road in Sidcup.
Police enquiries are ongoing to establish the circumstances.
No arrests have been made following the incident.
A spokesperson for Metropolitan Police said: “An investigation is under way after a 17-year-old boy was stabbed in south-east London.
“At 14:56hrs on Tuesday, 10 October, police were called to a south London hospital after the boy self-presented with a stab wound.
“His next of kin has been informed and await an update on his condition.
“Following enquiries by officers, it’s believed he was assaulted a short time earlier in Chislehurst Road in Sidcup.
“Enquiries are ongoing to establish the circumstances. There have been no arrests.
“Any witnesses are asked to call 101 quoting CAD 4609/10Oct.”