Emergency services were called to reports of a man found with a stab wound in his leg at the Admiral Steps at 11.10pm, police said.
The man was taken to the Royal Sussex County Hospital in Brighton for treatment.
He is in a serious but stable condition, confirmed police.
A police spokesman said: “Police are searching for two suspects after the victim said that two men got out of a car and one of them stabbed him before driving off.”
Any witnesses or anyone with information, report online or call 101 quoting 1640 9/10.