Luna was last seen wearing blue jeans, a white T-shirt, and Ugg boots.
She is described as 5ft 2in tall, of medium build, with curly hair.
A spokesperson for South Wales Police said: “We are appealing for information to locate 13-year-old Luna, who is missing from her home in Cardiff.
“Luna is 5ft 2 with a medium build and has curly hair. She was last seen wearing blue jeans and a white t-shirt with Ugg boots.
“She was believed to be in the Barry area in the early hours of this morning.
“Have you seen her?
“Please make contact – give reference 2600348069”.
Anyone with information is asked to contact police quoting reference 2600348069.
Members of the public can contact South Wales Police online or by calling 101.
