Potassium ion (K) channels have been observed in diverse viruses that infect eukaryotic marine and freshwater algae. However, experimental evidence for functional K channels among these alga-infecting viruses has thus far been restricted to members of the family which are large, double-stranded DNA viruses within the phylum . Recent sequencing projects revealed that alga-infecting members of , another family within this phylum, may also contain genes encoding K channels. Here we examine the structural features and the functional properties of putative K channels from four cultivated members of . While all four proteins contain variations of the conserved selectivity filter sequence of K channels, structural prediction algorithms suggest that only two of them have the required number and position of two transmembrane domains that are present in all K channels. After in vitro translation and reconstitution of the four proteins in planar lipid bilayers, we confirmed that one of them, a 79 amino acid protein from the virus Tetraselmis virus 1 (TetV-1), forms a functional ion channel with a distinct selectivity for K over Na and a sensitivity to Ba. Thus, virus-encoded K channels are not limited to but also occur in the members of . The large sequence diversity among the viral K channels implies multiple events of lateral gene transfer.