The two victims, aged 18 and 20, were walking along Kings Road Arches at about 3.45am on Tuesday (April 25), when police said they were approached by two women who attacked them.
Police said the 18-year-old was pushed to the ground, kicked, punched and then hit over the head with what was believed to be a glass.
Her Samsung mobile phone and handbag were stolen, officers said.
She was taken to the Royal Sussex County Hospital, Brighton, for treatment for cuts to her head and ear.
The two suspects reportedly ran off towards the i360 tower.
The first women was described as aged between 16 and 19, with long blonde hair extensions worn in a high ponytail, false eyelashes and fake tan on her legs, wearing a long sleeved black top, short black shirt and black trainers.
The second woman is described as aged between 16 and 18, with dark brown hair and was wearing a black dress and a long green cardigan.
Detective Constable James Botting said: “This appears to be a vicious and unprovoked assault and robbery on two victims. If anyone saw what happened or recognises the suspects please get in touch.”
If you were in the area at the time and witnessed the incident, report here with the reference 99 of 25/04.