The attack took place on Shelley Road at the junction with Salisbury Road at around 1.10 am.
A 37-year-old man was taken to hospital for treatment.
Detective Inspector Lee Simmonds said: “The suspect was described as a slim black man wearing a brown T-shirt and jeans.
“There will be an increased police presence in the area while we carry out inquiries and to provide reassurance.”
Sussex Police are asking for witnesses to come forward and seek CCTV, dashcam, doorbell or mobile phone footage to help with the investigation.
Information can be reported to Sussex Police online, quoting serial 76 of 11/08.
