The 13-year-old was attacked at around 1.20pm on Saturday at The Centre in Livingston.
She was in Poundland when a man approached from behind and indecently assaulted her.
He then left and ran off into the centre.
Inquiries are ongoing to trace this individual and officers have asked anyone who recognises him to come forward.
He is described as white, in his 40s, around 6ft with a slim build. He was wearing a light-grey shirt with the sleeves rolled up, a grey and navy T-shirt, dark blue jeans and black trainers.
Detective Constable Jo McCall, of Livingston CID, said: “This was a frightening attack on a young girl, which left her incredibly upset, though, thankfully, uninjured.
“We are eager to trace the man responsible as soon as possible and I would ask anyone who was in The Centre or Livingston Designer Outlet during the afternoon of Saturday and saw anything suspicious, to get in touch with us.
“Customers from these shopping centres may have dashcams attached to their vehicles and may have recorded the suspects.
“I would ask everyone to check their footage and if you believe it’s captured anything relevant then please contact police immediately.”