The Metropolitan Police were called at 2.20pm on September 23 with reports of an assault with a knife on Paddington Green.
Officers attended the scene alongside the London Ambulance Service, where a 16-year-old boy was found suffering a wound to his arm.
He was taken to hospital with non-life changing injuries and has since been discharged.
A short while later, officers arrested a 17-year-old boy on Edgware Road on suspicion of causing grievous bodily harm and possession of an offensive weapon.
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He remains in custody at this stage.
Anyone who witnessed the incident should call 101, quoting CAD 3914/23SEP.
To remain 100 per cent anonymous or contact the independent charity Crimestoppers on 0800 555 111.