Police were called at 3pm on Monday, 6 September to Eynsford Road, junction with Green Lane, Ilford, to reports of a stabbing.
Officers attended with LAS and found a 14-year-old suffering from a laceration to her neck.
Her condition at this time is not thought to be life-threatening.
Officers arrested another female on suspicion of GBH with intent and have taken her into custody.
A crime scene is in place and enquiries are ongoing.