A 22-year-old man was left with a broken jaw and a cut face after he was attacked on a Sunday afternoon.
Police are appealing for information about the incident on Lavender Hill, near Clapham Junction, which took place at about 2pm on June 22.
They are keen to speak to a group of men and women who witnessed the attack.
Detective Constable Lisa Lambert, investigating officer, said: "The victim has been left with very serious injuries that, as well as having an immediate physical impact on him, will doubtless have an emotional impact too.”
The victim was sat at a bus stop, on the opposite side of the road to William Hill, when a man wearing a red T-shirt crossed over and squirted him in the face with water.
The man crossed back over the road to a group of people, including a man with a dog. The victim followed him and another man hit him on the back of the head, knocking him to the floor.
The group separated and headed off in different directions down Lavender Hill.
It is believed that some of the group regularly visit the William Hill shop.
London Ambulance Service took the young man to a west London hospital where he was treated. He has since been discharged from hospital.
No arrests have been made at this stage and enquiries are ongoing.
Anyone who has information relating to this incident is urged to call Wandsworth Critical Incident Investigation Team on 020 3276 2586 or Crimestoppers anonymously on 0800 555 111.