Hydrobaenus olfa n. sp. from Algeria is described and illustrated as male, pupal exuviae and larva. The new species is assigned to the genus Hydrobaenus Fries, 1830 according to morphological characters observable in all the three stages, it is easily separated from all the other known species of the genus because of the high number of anal macrosetae on anal lobe of pupa; this character state is actually unknown within all the known species of the Hydrobaenus group, whereas it is observed in other genera within Orthocladiinae as in the genus Propsilocerus Kieffer, 1923. For this reason the description of the genus Hydrobaenus must be emended. This character gives additional evidence to the phylogenetic relationships between the primitive genera of Orthocladiinae.