Police were called to Monument View, Brighton, around 9.30pm on Saturday, August 24, after reports of a fight.
Police said a man in his late teens suffered serious head injuries with what was believed to be a hammer. He was taken to hospital for treatment and later discharged.
A police spokesman said: “A 23-year-old man and an 18-year-old woman were both arrested on suspicion of common assault and grievous bodily harm with intent. The pair were released on police bail until October 2 while the investigation is ongoing.
“We would like to hear from any outstanding witnesses and ask anyone with information to report it to us either online or call 101 quoting 1472 of 24/08.”