Police have appealed for witnesses following the accident on the A483 at Ardleen.
The crash happened at about 4.30pm on Saturday.
Three vehicles were involved – a blue Ford Fusion, a black Vauxhall Astra and a Morrisons lorry.
Dyfed Powys police said a 77-year-old man sustained fatal injuries and died at the scene.
Several casualties were also taken to hospital with non-life-threatening injuries.
Anyone travelling along this road at the time of the collision who has dashcam footage, or saw the collision or the vehicles involved, is asked to contact the Serious Collision Investigation Unit, at Dyfed Powys Police on 101, texting the non-emergency number on 07811 311 908.