The 22-year-old victim was walking from Union Street to Marshall Street in Larkhall at around 1.10am on Saturday when a man grabbed her.
The woman struggled with the man and managed to run away. Police said she did not require hospital treatment after the attack but has been left badly shaken.
The suspect is white, slim and was wearing a grey tracksuit.
Officers are checking CCTV in the area and particularly want to speak to a man who was walking a dog and spoke to the woman as she passed Central Bar on Union Street.
Detective Sergeant Charles Limond urged witnesses to call police on 101.