Alyssia – who is from the Toll Bar area of Doncaster – was last seen leaving her home address on foot at around 4pm on Friday, August 16.
She is believed to have links to the Manchester area.
Alyssia is described as having an olive skin tone, long dark brown hair and is around 5ft 3in with a slim build.
She is believed to have been wearing tiger print trainers with a white sole and carrying a black handbag when she went missing.
If you have any information which may help officers find her, please call 101 quoting incident number 710 of the 16 August 2019.