Heather Foggo, 48, became unwell during the Mighty Deerstalker event in the Scottish Borders on Saturday evening.
She was taken to Borders General Hospital in Melrose by emergency crews but did not survive.
A spokesman for Rat Race Adventure Sports, which organised the event, said they were "extremely saddened".
"The participant became unwell at 17:56, approximately one mile into the event, close to our event control station and at the location of one of our safety marshals," he added.
"A member of our medical team was on the scene within three minutes as we implemented our response protocols.
"They were assisted by an off-duty paramedic until the arrival of the Scottish Ambulance Service."
The spokesman said the company would offer its "full assistance" to the authorities.
The 10-mile (16km) event, billed as the UK's biggest night race, was being held near Traquair House, at Innerleithen in Tweeddale.
Two thousand people took part in Saturday's event, which began at 17:30.
Police Scotland said there were no suspicious circumstances surrounding the death and a report would be sent to the procurator fiscal.
A woman who died after falling ill during a nighttime cross country race was a nurse from Morpeth in Northumberland.