The serious accident happened shortly after 12pm and the A27 is expected to remain closed for up to three hours.
A West Sussex Fire & Rescue Service spokeswoman said: “We were called at 12.14pm today (Tuesday, May 3), to an accident on the westbound carriageway of the A27 in Arundel.
“It involved one vehicle which went into a tree. One female was released by us from the car using hydraulic rescue equipment.
“We also assisted the ambulance service with CPR.
“The A27 is currently closed eastbound and westbound and is expected to remain closed for three hours.
“Police are diverting traffic up the A29 and back down through Arundel.”
Sussex Police, in a statement, added: “Emergency services were called on May 3 at 12.19pm to a serious collision where a car had hit a tree on The Causeway A27 at Arundel.
“The road has been closed in both directions between Ford Road and Mill Road near Fontwell while emergency services are on scene.”
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