The landslide pushed a lorry into a river and uprooted trees between Carmarthen and Cardigan, blocking the road.
Corey Sharpling, 20, from Newcastle Emlyn, was killed at the scene at around 1700 GMT, local media reported.
Flood warnings were in place throughout the region on Saturday as rivers burst their banks flooding houses and streets but ended on Sunday after water levels started to recede.
Carmarthenshire, Ceredigion and south Powys were some of the worst hit areas.