The collision, which is believed to have involved two cars and a motorcycle, occurred on the anti-clockwise carriageway between Junctions 5 and 6 around 5.30pm.
Ambulance crews attended the incident but despite the best efforts of the paramedics, the motorcyclist, who is believed to be in his 30s, was sadly pronounced dead at the scene.
The air ambulance was also in attendance.
The M25 is still closed in both directions while the investigation into the circumstances of the collision are carried out.
The M26 has also been closed and parts of the A21 to assist with the fatal collision investigation
Police are appealing for anyone who witnessed the collision, or who saw the vehicles being driven prior to the incident, particularly if you have dashcam footage, to contact us straight away via webchat on our web site or by calling 101 and quoting reference PR/P21080881.