Police were called to the station today (June 13) at around 10.10am to reports of a casualty on the tracks.
The person was pronounced dead at the scene.
British Transport Police (BTP) officers are not treating the incident as suspicious and a file is being prepared for the coroner.
Thameslink lines were closed between London St Pancras and Luton as emergency services safely carried out their work.
All lines have since reopened just after noon today, however major disruption is still estimated to last until 3pm.
National Rail has advised that people delay their Thameslink journeys as long as possible due to the delays.
A BTP spokesperson said: “Police were called at around 10.10am today (13 June) to reports of a casualty on the tracks at West Hampstead Thameslink station.
“Officers responded with paramedics, but sadly a person was pronounced dead at the scene. The incident is not being treated as suspicious and a file will be prepared for the coroner.”
If you feel affected by what has happened and wish to find support, you can contact the Samaritans on 116 123 via phone for a confidential chat or email jo@samaritans.org