Three fire engines were sent from Henfield, Horsham and Worthing to the A24 just before 5pm, Sussex Fire and Rescue said.
A car was on fire in a slip road on the southbound carriageway, just past the Buck Barn crossroad.
The car was seriously damaged by the fire, which was extinguished using a high pressure hose reel.
Firefighters left the scene, leaving the incident with police, at 5.54pm.
No injuries were reported.
PC Tom Van Der Wee tweeted: ‘We’re on scene with @WestSussexFire at a car fire on the A24.
‘Thankfully the occupant of the car is uninjured and the fire is now out.’