Berthe was last seen on June 29 at around 7 p.m. and was wearing a black coat and grey and green shoes. She is known to hang out in Lachine and nearby neighbourhoods.
She is English-speaking and has brown hair and brown eyes. She is just over five feet tall, weighs 114 pounds, and has a lip piercing.
Police said Berthe has a mental age of eight, and she does not have any money or an Opus card.
People with information can call the Montreal police's anonymous tip line, Info-Crime, at 514-393-1133.