The accident happened just before 1900 GMT on the northbound carriageway between junctions 25 and 26 to the south of Taunton.
A spokesman for the Devon and Somerset Fire Service said: "We can confirm that one fatality has been discovered."
Several other people were treated at the scene and two people were taken to hospital, one with head injuries.
About 20 passengers are believed to have suffered injuries in the accident.
Motorway closed
The accident happened about a mile north of the Taunton Deane service area and the motorway was closed in both directions.
Thirteen ambulances from across the region attended the accident, which spilled on to both carriageways.
Fire crews left the scene at about 2300 GMT after making about eight vehicles safe.
"There were no persons trapped in the vehicles but there were several other casualties with minor injuries, who were treated on at the scene," the fire service spokesman added.
Avon and Somerset Police said the motorway was expected to remain closed for some time.
A force spokesman said: "Police are urging motorists to use alternative routes and avoid the area where possible.
"It is expected the motorway will be closed for a number of hours."
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