We present the description of Mallomonas teres, sp. nov., from three localities, two peat-bogs in Northern Sweden and a peat-bog pool in the Polessian Nature Reserve, Northern Ukraine. The sites ranged from acidic to neutral pH, were low in conductivity, and contained other pH-indifferent or low-pH chrysophyte taxa. According to the scale structure M. teres, sp. nov., belongs to the section Heterospinae within the genus. The most distinctive scale character discerning M. teres, sp. nov., from the other species of the section Heterospinae is the distal part of the scale, which has a smooth dome and numerous parallel curved ribs. Mallomonas teres, sp. nov., is probably distributed at least in temperate, acidic European localities. Its rare occurrence could be a result of a poor dispersal capacity combined with a low growth rate.