The incident happened in Sutton Avenue, Peacehaven at around 5.30pm.
Police cordoned off the street following the incident and, according to reports, the air ambulance landed on the Dell playing field in Peacehaven as emergency services attended the scene.
A Sussex Police spokesperson said: “We’re investigating a reported stabbing which occurred in Sutton Avenue, Peacehaven, around 5.30pm on Friday, May 19.
“A 19-year-old man from Brighton sustained serious but not life-threatening injuries and was taken to hospital for treatment.
"We are treating this as an isolated incident with no threat to the wider community. Our enquiries are ongoing and we’re urging anyone who witnessed the incident – or anyone with information – to contact us online or call 101, quoting reference 47230092872.”