A new species, Mallomonas vietnamica Gusev, Kezlya & Tran, sp. nov., is described from Vietnam based on silica-scale morphology elucidated by transmission and scanning electron microscopies. The species belongs in section Mallomonas Perty and is characterized by large oval scales with a wide and deep dome, which is recessed from the scale perimeter. The domes are shifted to the right side of scales and oriented at different angles to the scale's longitudinal axis. Bristles of the newly described taxon are characterized by a distinctly hooked foot. Scales of M. vietnamica Gusev, Kezlya & Tran, sp. nov. were found at only one locality in Hanoi, at the border of Ba Vi National Park in the freshwater Da River (Sông Đà).