ABSTRACT. – Since 2004, an unknown diatom taxon has been spreading through an increasing number of French watercourses. Our findings led us to describe it as a new species, Achnanthidium druartii sp. nov. This taxon can easily be differentiated from other morphologically closely related species like A. convergens, A. deflexum, A. japonicum, A. latecephalum, A. pyrenaicum and A. rivulare with both light and electron microscopy since its valve is longer, its stria density is lower and it has a lanceolate shape with subrostrated apices (never capitated). The assessment of its ecology was made through physicochemical analyses carried out on sites that are part of the river monitoring quality networks. The new taxon is an epilithic taxon, often observed in well mineralized waters running through sedimentary rock and of quite low trophic and saprobic levels. Many of the sites are large rivers and navigation channels. Considering the increasing number of stations where it was recorded and since it is only recently recorded in rivers, this taxon can be considered as invasive.