Erythropsidinium (formerly Erythropsis) is a genus of dinoflagellates (a type of unicellular eukaryote) of the family Warnowiaceae.

Erythropsidinium cells are a relatively large by dinoflagellate standards (50-120μm in longest dimension) and possess two flagella and two unusual organelles, the ocelloid and the piston.

The piston is capable of repetitive and dramatic contractile motion; its function is not clear, but roles in locomotion, prey capture, and defense have been suggested.

[3] Erythropsidinium occurs in marine plankton and is found in warm or tropical waters, reported in all oceans and in the Mediterranean Sea.

[3] The genus was originally described under the name Erythropsis by Richard Hertwig in 1885 based on samples from the Gulf of Naples.