Police were called to the west-bound carriageway, at the junction with Steyning Road, at 12.36am.
A 39-year-old woman driving an Audi had collided with the 42-year-old pedestrian, who was in the carriageway.
The pedestrian died at the scene from her injuries.
The A27 was closed for more than three hours while the emergency services attended the scene and police would like to thank motorists for their patience.
Sussex Police is appealing for witnesses and anyone with information can call 101 quoting Op Tallow.
Did you witness this incident?
You can call the newsdesk on 01903 282378 or email [email protected].