North Wales Police said the pedestrian died following a collision with a lorry at at the UPM plant in Shotton, at around 11:30 GMT on Monday.
A man is currently in custody helping police with inquiries.
The site is now closed as police and Health and Safety Executive investigators interview witnesses and staff.
General manager David Ingham said: "Our immediate thoughts are with the individual's family and colleagues.
"We will provide full support to all staff involved and investigate fully with all parties."
An investigation is under way after a man died at paper mill in Flintshire.