Police said emergency services were called to the junction of Poling Street and the A27 Arundel Road at around 11.15pm on Wednesday, 15 July, following reports of a collision involving three vehicles.
A Sussex Police spokesperson said: “Sadly, a woman in her 20s from Sidlesham, who was a passenger in one of the vehicles, was pronounced deceased at the scene.
“Her next of kin have been informed and will be supported by specially trained officers. Our thoughts remain with them at this difficult time.
“The road was closed for several hours while emergency services responded and enquiries were carried out.
“An investigation into the circumstances of the collision is ongoing, and officers are keen to hear from anyone who witnessed what happened or captured any relevant CCTV, dash cam or mobile phone footage.
“Anyone with information is asked to call 101 or email [email protected] quoting Operation Stanbridge.”
A West Sussex Fire & Rescue Service (WSFRS) spokesperson added: "Joint Fire Control sent four fire engines from East Preston, Littlehampton and Worthing to the scene, as well as a heavy rescue tender from Worthing.
"Firefighters assisted Sussex Police in making the scene safe, before booking away at 12.40am."
The A27 has reopened open in both directions between A284 Crossbush and A280 Clapham, according to National Highways.

1. Arundel
Sussex Police reported a 'serious collision' on the A27 at Arundel. Photo: Sussex News and Pictures Photo: Sussex News and Pictures