They believe Marie-Pierre Drouin-Ulysse, 13, ran away from a youth center in Ahuntsic.
She has been missing since July 22.
Police spokesperson Daniel Richer said she may be in Saint-Michel or Hochelaga-Maisonneuve, because those are areas she frequented.
Police fear for her safety.
Anyone with information should call Info-Crime at (514) 393-1133.