The 22-year-old woman was walking from Union Street to Marshall Street in Larkhall, South Lanarkshire, at around 1.10am today when a man grabbed her.
The woman struggled with the man and managed to run away. Police said she did not require hospital treatment over 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 said: "Enquiries are at an early stage and officers are working in the area to gather more information on this incident and the man responsible. We are also gathering and checking CCTV footage to establish a better description of the suspect.
"The young woman did not require hospital treatment but is badly shaken as a result of this assault.
"I would urge any witnesses to this crime, or anyone who may have seen the man responsible in the local area around the time of the incident to come forward to police as they may have information that is vital to this investigation.
"In particular, I would like to speak to a man who spoke to the victim as she walked past the Central Bar in Larkhall. He was walking a small white dog at the time."