British Transport Police attended the incident at Harrow-on-the-Hill Underground Station just before 11am. The girl has been taken to hospital with what is thought to be a stab wound.
Her condition is not believed to be life threatening.
The station has been cordoned off and remains closed.
A BTP spokesperson said: “Officers were called to Harrow on the Hill Underground station at 10.56am following reports of a serious assault.
“Paramedics also attended and a 16-year-old girl has been taken to hospital with an injury consistent with a stabbing.
“Thankfully, this is not believed to be life-threatening.
“The station is closed and cordons are currently in place at the scene while enquiries continue.
“Anyone who witnessed what happened can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 170 of 20/10/20.”
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“A person was treated at the scene for a leg injury and taken to a major trauma centre as a priority.”
There are some delays on the Metropolitan Line following the incident.
In an earlier tweet, Greenhill Police said: “Due to an ongoing investigation, Harrow On The Hill tube station is currently closed and there is disruption to the Metropolitan Line. We will keep you updated.”