Chytrids are early diverging lineages of true fungi that reproduce with posteriorly uniflagellate zoospores. In aquatic ecosystems, parasitic chytrids of algae have important ecological roles by influencing the population dynamics of phytoplankton and transferring nutrients and energy from inedible algae to zooplankton via zoospores. Despite their ecological importance, information on parasitic chytrids is lacking in the current systematics of chytrids. Here, we investigated a novel chytrid culture KS100 that parasitizes the green alga, Microglena coccifera (Volvocales). A cross-inoculation experiment revealed that KS100 infection was specific to the genus Microglena. Thallus morphology of KS100 is characterized by spherical or subspherical zoosporangium, which becomes slightly angular during zoospore discharge, 2-3 small and inoperculate pores from where zoospores are discharged, and rhizoids branching at the base that extends in a fan-like shape. This combination of characteristics was distinct from any other known chytrids. In molecular phylogeny, KS100 was placed in the order Rhizophydiales and was distinguished from any known families in the order. Zoospores of KS100 possessed a kinetosome-associated structure whose morphology and positioning were unique among the Rhizophydiales. Based on these results, we describe this chytrid as Collimyces mutans gen. et sp. nov. in the new family Collimycetaceae.