The incident happened in Princes Road at the junction with Darenth Road just before 10.45pm yesterday (May 25).
Officers attended the scene alongside South East Coast Ambulance Service.
The rider of the e-scooter, a 21-year-old man, was taken to hospital in London, where he remains in a serious condition.
Kent Police said it is investigating the circumstances and has appealed for witnesses to come forward.
The force has asked anyone who saw the collision, or who may have seen the vehicles beforehand, to contact the force.
The MP for Dartford, Jim Dickson, said he is “deeply saddened” after hearing about the incident.
The MP added that the road should reopen shortly.
Mr Dickson said: “I am deeply saddened to hear of the serious accident on Princes Road last night.
“Police are currently investigating the circumstances and my thoughts go out to the person who has been very badly injured and their family and friends.
“The road should be open again shortly.”
