A new diatom species is described from the Willamette River, Oregon, USA. The characteristics of this taxon at the light microscopy (LM) and scanning electron microscopy (SEM) levels favor its placement in the genus Adlafia Moser, Lange-Bertalot et Metzeltin. Thus, the name Adlafia multnomahii Morales et Le is proposed. Analysis of additional samples currently deposited in the Academy of Natural Sciences of Philadelphia (ANSP) Diatom Collection yielded other populations of the new taxon from Arkansas, California, Connecticut and New Hampshire. The new taxon is characterized by a lanceolate shape with conspicuously capitate to rostrate ends, distal and proximal raphe ends deflected in opposite directions, round areolae occluded by a perforated velum located on the outer periphery of the areola, and closed girdle bands with one row of areolae, which becomes biseriate toward the apices of the valves. A comparison of the new taxon with other morphologically closely related taxa and its ecological requirements are discussed in the light of existing data and literature.