The incident happened on the Northern line just before 11.20am today (August 14).
Officers and London Ambulance Service crews attended, however, the person was pronouncded dead shortly after.
Old Street station Northern line closed after incident: Live
The death is not being treated as suspicious and a file will be prepared for the coroner.
The line running Moorgate and Finsbury Park is currently closed as the incident is dealt with.
Disruption is expected until 12.45pm.
A spokesperson for London Ambulance Service said: “We were called at 11.22am today (14 August) to reports of a person hit by a train at Old Street Underground Station, EC1Y.
“We sent resources to the scene, including an ambulance crew, an incident response officer and members of our hazardous area response teams. We also dispatched a trauma team in a car from London’s Air Ambulance.
“Our first paramedic arrived on scene in less than four minutes.
“Very sadly, despite our best efforts, a person was pronounced dead at the scene.”
A spokesperson for British Transport Police said: “Officers were called at around 11.25am today (14 August) to Old Street Underground station following reports of a casualty on the tracks.
“Officers responded with paramedics, however sadly a person was pronounced dead at the scene.
“The death is not being treated as suspicious and a file will be prepared for the coroner.”
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This is a breaking news story. More to follow.