The girl was walking along the footpath between Middleton Avenue and Loaninghill Road in Uphall when a man approached her.
The man touched the 15-year-old inappropriately, and police have now launched an investigation to trace him, including checking CCTV and speaking to locals who live in the area.
The suspect has been described as a white male who, at the time, was wearing a face mask, a hooded top with dark trousers and was carrying a multi-coloured rucksack.
Detective Inspector Stephen Wood, of Livingston Police Station, said: "Although the girl did not require medical treatment, she was left very shaken by what happened.
"Since the incident, officers have been carrying out enquiries in the surrounding area, including checking CCTV and speaking to local residents.
"If you were in the area at the time, please think back and get in touch with us if you have any information which may be able to assist our investigation.
"I am keen to hear from anyone who has private CCTV, doorbell camera footage, or motorists with dash cam footage.
"This footage could prove very useful to our enquiry.
"I am also appealing to anyone who may have seen anything suspicious, or a man matching this description, to come forward."
Anyone with information is asked to contact Police Scotland via phone number 101, quoting reference number 0664 of Tuesday, 21 June, 2022.
Alternatively, details can be provided anonymously to Crimestoppers on 0800 555 111.