Emergency crews were called to Wacker Quay, near the village of Antony, after concerns were raised for a person in the water at about 6pm on Saturday.
Wacker Quay provides public access to the River Lynher.
Devon and Cornwall Police said the girl, from South Devon, was recovered and given emergency treatment, but was confirmed dead at the scene.
Her death is not being treated as suspicious.
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A spokesperson for the force said the girl’s next of kin had been informed and will be supported by officers.
They added: “A file will be prepared for the coroner.”
