Kelsey Wardrop, 15, from Foundry Wynd in Kilwinning has been missing since out with friends yesterday.
Her family say they "fear the worst" as the teenager was last seen yesterday evening around 6pm.
An un-confirmed sighting of Kelsey was told by her pals, they say she was boarding a bus at Kilmarnock Bus Station - heading towards Stewarton around 10pm last night.
Kelsey’s sister, Stacie Wardrop, 20, told the Irvine Times: “This is very out of character for Kelsey. She is always with her friends and keeps in contact a lot.“
"Kelsey is a very honest girl, if she is going to be late then she will usually phone or text from a friend’s mobile phone to us let us know.
"But at the moment no friends have said they were with her or have seen her.”
The Kilwinning Academy pupil is described as around 5ft tall, skinny with dark blonde hair, and was described as wearing black leggings, and a dark top at the time of her disappearance.
The family have reported Kelsey's disappearance to police who are investigating.
A spokesman for Police Scotland said: “We are aware of Kelsey’s disappearance and inquires are at an early stage. Anyone with any information should contact the police by calling 101.”