The police's serious crash investigation unit was sent and part of the motorway was closed.
The force said the coastbound M20 was reopened at 08:15 BST and the London-bound carriageway opened at midday.
A spokesman added: "A 25-year-old man from the Maidstone area was pronounced dead at the scene.
"His next of kin have been informed.
"Any witnesses to the incident are asked to call the serious collision investigation unit."