The accident happened on the A40 between Manordeilo and Llandeilo at around 10.30am on Thursday.
Dyfed-Powys Police said the driver of a grey Ford Fiesta was pronounced dead at the scene.
The police are now calling on witnesses to come forward.
“Ambulance attended the scene but tragically the woman driving the car died,” the police said in a statement. “Next of kin and HM Coroner have been informed.”
The road was closed for several hours so investigations could be carried out.
It added: “Police are urging anyone who witnessed the collision, or was driving along that stretch of road around the time of the incident, to phone them on 101.”