The man, in his 50s, is believed to be from the Maidstone area of Kent.
The clockwise carriageway leading to the Dartford crossing will remain shut until further notice following the crash between junction 30 and junction 31, police said.
The arrested female driver, aged in her 30s and from Romania, remains in custody and is being interviewed.
Paramedics attended and both carriageways were shut to allow an air ambulance to land. Police said they were now carrying out an investigation.
The slip road from Grays on to the clockwise carriageway is also closed.
Motorists are advised to seek an alternative route, although the anti-clockwise carriageway has now reopened.
Traffic congestion is very heavy but vehicles initially caught at the scene of the collision have been released, police said.
A lorry driver from Romania has been arrested after a motorcyclist died in a collision on the M25 in Essex.