Phalansterium is a genus of single-celled flagellated organisms comprising several species, which form colonies. Phalansterium produces tetraspores.

Phalansterium is hard to classify; it has a distinctive ultrastructure of its pericentriolar material. Molecular evidence places it in the Amoebozoa.

It has been suggested that it is similar to the ancestral eukaryote.

Genus Phalansterium Cienkowsky 1870