Achnanthes armillaris is a small, monoraphid diatom in the family Achnanthidiaceae, attached to submerged substrates by a mucilage pad at one pole of its frustule. It inhabits freshwater streams, rivers, and lakes, forming part of the epilithic and epiphytic diatom community. Like other Achnanthes species, it serves as a bioindicator for water chemistry and flow regime in freshwater monitoring programs.