British Transport Police said officers were called to South Croydon station at 9.13am following reports of a casualty on the tracks.
A spokeswoman added: “Paramedics also attended however sadly a person has been pronounced dead at the scene. Officers are now working to identify the person and inform their family.”
The incident is not being treated as suspicious and a file will be prepared for the coroner, police said.
All lines were blocked but have now begun to reopen, according to Southern Rail.
A spokesman added: “Services will continue to be disrupted as trains and crew are displaced.
“We’re working hard to get back to our schedule.”
Major disruption is expected to Thameslink and Southern services until midday.