Sussex Police said emergency services were called around 11.40am.
They were responding to reports of a driver 'suffering a medical episode', police said.
A spokesperson added: "The man was passed into the care of paramedics."
According to an eye-witness, at least three ambulances responded to the incident on the Bridge Road (Tesco) roundabout.
A spokesperson for South East Coast Ambulance Service said: "Ambulance crews attended the scene and a person was treated at the scene before being taken to Worthing Hospital in a life-threatening condition."