The incident happened between junctions 28 and 27 just before 8.30am today (July 16).
Officers were called to the scene and found one person injured.
They were taken to hospital for treatment where their condition was assessed as serious.
Closures were put in place which caused severe delays and congestion leading to the Dartford Crossing.
The road has now fully reopened.
A spokesperson for Essex Police said: “We were called to a collision involving three vehicles on the M25, between junctions 28 and 27, at just before 8.30am today (Wednesday 16 July).
“One person has been taken to hospital in a serious condition.
“The road is now fully reopened.
“If have any dashcam footage or witnessed the collision and haven’t already spoken to us, then please contact us.”
