Thecate amoebae play important roles in terrestrial and aquatic ecosystems. This study introduces a novel thecofilosean amoeba from Arctic and Antarctic sea sediments. Phylogenetic analysis based on the 18S rDNA sequence places it in the family Chlamydophryidae (order Tectofilosida, class Thecofilosea). However, the novel organism exhibits a significant genetic divergence and distinct morphology from its closest relatives, prompting us to erect the novel genus Katarium with its type species Katarium polorum. K. polorum is a consumer of diatoms and prokaryotes, indicating an important role in nutrient cycling in the polar marine food webs.