Based on light, scanning and transmission electron microscopy observations, a detailed description of a new marine fultoportulate diatom Sinerima marigela gen. et sp. nov. is presented. This new taxa is rare (<3% of the thanatocoenosis) in surface sediments in the Bering and Chukchi seas, but appears to have an association with high spring sea ice concentrations. The new monotypic genus is distinct due to its 1) lack of rimoportulae, 2) one-layer valves with marginal pseudoloculi, 3) characteristic velum composed of a cluster of short tubes, and 4) central part without either central fultoportulae or an annulus. This set of characters, especially the lack of rimoportulae, makes S. marigela unique and easily distinguishable from other fultoportulate diatoms.