The 16-year-old, who cannot be named due to his age, was on trial accused of murder and manslaughter following the death of Aria Thorpe. A jury returned verdicts of not guilty for both charges at Bristol Crown Court earlier.
A pathologist previously told the court Aria died of a single stab wound to her chest at her home in Weston-super-Mare on 15 December 2025.
After her death, the nine-year-old's family paid tribute to her "lively and bubbly" personality.
The accused had told jurors he and Aria had been playfighting when he accidentally stabbed her.
He told the court he was holding the knife and "waving" it around.
"I was going to try and make her flinch, so I leaned forward like I was fencing," he said.
After Aria suffered the fatal injury, the 16-year-old left the house and headed for Worle Railway Station. He said he left because he got "scared" and "panicked".
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