A woman waiting at bus stop was forced at knife-point into a side street where she was raped in a front garden.
The 43-year-old victim was making her way home after becoming separated from friends in Tooting, south London.
The man threatened the woman with a knife while they walked from Tooting Broadway, along Mitcham Road and into Ashvale Road, where he attacked her.
The suspect then walked back to Mitcham Road where he boarded a N133 bus towards Streatham, police said.
The rape, which is thought to have happened in the garden of a house in Ashvale Road, happened at about 0235 BST on Friday.
Det Insp Dave Cannon, said that the victim had come into contact with the suspect while they were in the pub, The Tramshed.
"We believe that she became separated from her friends and the suspect threatened her with a knife if she did not do what he wanted," he said.
The man is described as being of Eastern European or Mediterranean appearance, in his early 30s, 5ft10in (1.77m) with short dark hair.
Anyone with information about the attack is asked to contact police.