The ED at the Marlborough Road hospital was closed between 5pm and around 10.10pm, after a police officer attended with medical symptoms related to the Amesbury Novichok poisonings.
The officer has been taken to Salisbury District Hospital for treatment.
The emergency department was shut and cleaned to Public Health England guidelines.
A South Central Ambulance Service crew was turned away from the ED at around 8.30pm. Members of the public who arrived at the doors of the ED were told to visit emergency units in Cirencester or Bath.