The A280 has been closed in both directions between Worthing and Angmering.
UPDATE: This story has now been updated after the motorcyclist sadly passed away: A280 accident: Police appeal for witnesses after motorcyclist killed between Worthing and Angmering
A spokesman for Sussex Police said a motorcyclist was seriously injured when he came off his bike at the A280 junction with the A27 near Angmering just before 4.40pm this afternoon (June 17).
No other vehicle is reported to have been involved, the spokesman said.
Emergency services are at the scene, the police said, and the A280 was closed at the junction with the A27 down to Water Lane.
The spokesman added the investigation was at an early stage.
Pictures show at least four police vehicles on the scene.
The emergency services have been contacted for more information.
Another accident in Worthing is causing traffic along the seafront: Gridlock in Worthing and Shoreham as A259 closed following accident