Mya Baker was last seen leaving her home in Birtinya on Queensland's Sunshine Coast at 6.45pm on Wednesday.
She left to go to a nearby park wearing a black T-shirt with red lines on it, black bike pants, and was shoeless.
Mya has not contacted her family or been seen since.
"It's certainly out of character, she's never spent a night away unknown from her family," Detective Inspector Chris Toohey said.
Det Insp Toohey said Mya's family is devastated and extremely worried about her.
A 20-minute window between the time Mya left her home at 6.45pm and when her brother went looking at 7.05pm is a key part of the police investigation.
Another line of inquiry is Mya's online activity and who she may have organised to meet at the park the night she disappeared.
Det Insp Toohey said police are gravely concerned about Mya's whereabouts but vowed to do everything possible to find her.
Mya is described as having brown, shoulder-length hair, blue eyes and is about 165cm tall.
Anyone with information should contact police.
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