The assault is alleged to have happened in the area of Palmer Rise in Livingston at about 7.30pm yesterday.
Officers said their inquiries are at an early stage and have urged anyone with information about the incident to come forward.
A Police Scotland spokesman said: "Police in West Lothian are investigating following reports of an assault on a 13-year-old girl.
"The incident took place on Palmer Rise in Livingston at about 7.30pm on Wednesday April 2.
"Inquiries are at an early stage and anyone who can assist with the investigation should contact Police Scotland on 101 or, alternatively, the charity Crimestoppers on 0800 555 111."