British Transport Police closed Queensbury Tube station in north-west London following the shocking incident at 12:50pm.
Officers responded to reports of a fight and found the victim, who has not been named, had been attacked with a knife.
Witnesses reported police and ambulances on the scene and roads closed around the Jubilee line station.
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One terrified onlooker said "armed police and helicopters" were on the scene.
Another wrote: "Station is closed, several police vans and coppers as well as a crowd outside the entrance."
Officers have not made any arrests and are appealing for witnesses to come forward.
A spokesman for the British Transport Police confirmed officers were at the station "following reports of a fight on a platform".
He added: "During this fight, one man suffered injuries consistent with being stabbed.
"His injuries are not thought to be life-threatening and he is receiving treatment in hospital."
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