Two unknown diatoms belonging to the genus Brachysira were found during a survey of the moss-inhabiting diatom flora in Canada, Greenland, Iceland and Spitsbergen. Detailed light and scanning electron microscopy revealed sufficient morphological differentiation in valve dimensions and shape, striae density, and ultrastructure for them to be described as new species. The description of Brachysira louiseboydiana sp. nov. and B. hutchisoniana sp. nov. brings the total number of Brachysira species globally to 132. Formal descriptions are presented, each illustrated by light and scanning electron micrographs, and the new species are compared with similar Brachysira taxa worldwide. Additionally, four previously described species which were also observed in the Arctic mosses are discussed. Our findings contribute to a better understanding of the Arctic terrestrial diatom flora.