Dyfed-Powys Police has confirmed a woman died after her car left the road near the Pont Abraham roundabout.
She was pronounced dead at the scene on the A483 last night.
Her next of kin have been informed.
The accident happened shortly after 22:00GMT.
The road was closed last night during investigations but was reopened just before 5.30am.
Fire crews from Ammanford and Pontarddulais were at the scene, along with Dyfed-Powys Police, two ambulances, an ambulance officer and paramedic supervisor.
A police spokeswoman said, "Police were called at 10pm on Wednesday to a report of a vehicle having left the A483 road just off Pont Abraham roundabout, Carmarthenshire.
"Sadly the female driver of the vehicle was pronounced dead at the scene."