Ptychagnostus is a genus of trilobites in the order Agnostida that lived during the Cambrian period.

Their remains are rarely found in empty tubes of the polychaete worm Selkirkia.

[3] Agnostus punctuosus Angelin, 1851 from the Ptychagnostus punctuosus zone of the Alum Shale (Drumian), Sweden (by original designation).

Official ruling on the conservation of accepted usage of A. punctuosus as the type species was given by the International Commission on Zoological Nomenclature, 1993.

Ptychagnostus affinis (Brøgger 1878) [4] was once considered a subspecies of Ptychagnostus punctuosus.Laurie (2008) [5] grouped punctuosus and affinis within Ptychagnostus, but preferred to place the closely related atavus within Acidusus.