The victim was a woman in her forties and the assault took place in the Iveagh Close area in Crawley at around 8pm on Monday.
According to police, the man appeared in front of her, pushed her inside a property and sexually assaulted her.
The man is described as being in his twenties, between 5ft 2in and 5ft 4in, of mixed race, slim build, with short or shaved hair and a moustache. He was wearing a green khaki jacket, believed to be a zip-up, and green camouflaged patterned trousers.
Detective Police Constable Michael Wyborn said: “This was a distressing incident for the victim and we are providing support for her during this time.
“We believe the suspect jumped over the garden wall when he left the property and we are appealing for anyone who may have seen him to get in contact with us.”
If you witnessed anything please report online or call 10 quoting reference 1266 of 06/11.