Police were called just after 2pm on October 20 by a woman who said she had been assaulted in Morden Recreation Park.
The woman was with her family, including two young children, when she was approached by the suspect who is described as in his late teens.
He spoke briefly to her before assaulting her.
The suspect was around 6ft tall and of skinny build, wearing a burgundy hoodie with white writing on it, blue tracksuit trousers and black trainers.
He ran away in the direction of Canterbury Road.
Police have released an E-fit of the suspect and are urging anyone who can identify him to come forward.
Detective Constable Tom Green, from the local policing team, said: “We know this assault will cause a lot of worry in the local community, especially as this woman was playing with her children at the time.
“I would ask you to take a close look at the E-fit. Do you recognise him, or the clothing that he was wearing?
“It is vital that we identify him.
“If you have any concerns, please speak with officers.”
Anyone with information is asked to call police via 101 reference CAD 3851/20Oct.
To remain anonymous contact Crimestoppers on 0800 555 111.