AbstractRhodelphidia is a recently discovered phylum within the supergroup Archaeplastida, comprising only two known representatives (Rhodelphis marinus and Rhodelphis limneticus). Despite its close phylogenetic relatedness to red algae, Rhodelphidia differ markedly by being nonphotosynthetic eukaryotrophic flagellates with gene‐ and intron‐rich genomes. Here, we describe a new freshwater Rhodelphidia species, Rhodelphis mylnikovi sp. n., strain Rhod‐M. It shows clear morphological differences with the two other Rhodelphis species, including larger cell body size, presence of two contractile vacuoles, short and blunt pseudopodia, absence of cysts, and tendency to cannibalism. 18S rRNA‐based phylogenetic analysis placed it sister to the freshwater species R. limneticus.