A robber held a knife to a 21-year-old woman's throat before stealing her handbag in central Swansea.
Police say although the victim was not seriously injured, she was left badly shaken by the ordeal.
The robbery occurred at about 0300 GMT on Tuesday as she was walking along Walter Road.
Police say they are looking for a white male in his early 20s who was wearing a dark coloured tracksuit with a hood pulled over his face.
The woman was approached from behind by the suspect who placed his hand over her mouth and told her not to scream, said police.
He then moved in front of her, removed his hand and held a knife to her throat and demanded that she hand over her handbag.
Once she did. the man ran off.
Det Insp Peter Doyle, of Swansea Central CID said: "This was an extremely frightening experience for this young lady, who fortunately was not seriously injured, but the ordeal has left her badly shaken.
"We need to identify and apprehended this man before he attacks again.
"In order to do this we need to enlist the help of the public. I would urge anyone with information which could help us in identifying this man to come forward.
"I would like to reassure the local community that we are doing everything in our power to arrest this man and bring him to justice."