More than 50 Portuguese Man O’ War have washed up at Cefn Sidan beach.
The jellyfish-like creatures can inflict painful stings, even when they are dead.
Beach walkers are being warned to be careful, not to walk barefoot on the sand and to keep dogs on leads.
Hundreds of the creatures have washed up on coastlines across south west Wales.
The Marine Conservation Society advises to leave them on the beach.
Carmarthenshire County Council executive board member for tourism Cllr Peter Hughes Griffiths said: “More than 50 of the stinging creatures have been found over a two-mile section of the beach. Rangers have erected signs to warn people entering the beach from Pembrey Country Park.”
Portuguese Man O’ War are usually found in the Atlantic and Pacific oceans, have purple tentacles which can reach up to 50 metres below the ocean’s surface.