The 37-year-old was attacked outside the Turtle Bay bar and restaurant in Southampton's Guildhall Square at about 21:00 BST on Friday, police said.
An eyewitness said the male attacker, who fled towards Above Bar Street after the attack, appeared to know the woman.
Police described it as a "rare and isolated" attack. The victim was taken to Southampton General Hospital.
A fire crew was also sent to the scene to wash away the acid from the pavement.
Hampshire police is yet to release information about the woman's condition.
Adrian Rodgers, assistant general manager at Turtle Bay, helped give the woman first aid, which included rinsing the woman's face and mouth, before the ambulance arrived.
A statement from the company added: "The lady was not a customer of Turtle Bay, however, the manager on duty provided immediate assistance and emergency first aid whilst the emergency services were called.
"When they arrived they immediately took her to hospital to continue her treatment.
"We hope that she makes a full and speedy recovery from the incident."
A woman suffered severe burns to her face, neck and arms when she had acid thrown over her during a night out.