The teen, who lives in Montreal's Lachine borough, told her mother that morning she was planning to eat supper at a friend's house.
According to police, she showed up at the friend's house at around 4 p.m but left soon after, saying she was heading to her father's.
Lamontagne goes by the name of Vicky.
She stands five feet tall and weighs 143 pounds. She has shoulder-length brown hair and brown eyes.
She was last seen wearing grey leggings, a short-sleeved black sweater with a white short-sleeved sweater underneath, as well as grey sandals.
She wasn't carrying any identification or money.
Anyone with information about the teen should call the Info-Crime line at 514 393-1133.