The teenager is described as being around 5ft 2ins tall, white with straight brown hair.
Avon and Somerset Constabulary have said they are becoming increasingly concerned for young Rosie, who was reported missing after getting into a Vauxhall Astra on October 31.
Rosie was last seen in the rural village of Holcombe Rogus, Devon and has connections to the Taunton and Bridgwater areas.
The teenager is described as being around 5ft 2ins tall, white with straight brown hair.
A police statement read: “Can you help us find missing Rosie, 15, who was last seen getting into a blue Vauxhall Astra, in Holcombe Rogus, at around 4.45pm on Friday 31 October.
“We are growing concerned for her welfare. If you know where Rosie is, please call 999 quoting reference 5225308184.”
It comes after police in London launched a search for two schoolgirls who went missing after leaving their home.
Mia, 9 and Olivia, 10, went missing in the Redbridge area on the morning of October 28.
The two young girls were eventually found safe, police said.
