During the Paleocene–Eocene thermal maximum (PETM), low to mid-latitude planktic foraminiferal assemblages were characterized by blooms of the surface-dwelling Acarinina. Among this group a new ‘excursion taxon’ is identified, Acarinina multicamerata n. sp. Previously, this taxon was lumped together with Acarinina sibaiyaensis El-Naggar. Considering that A. sibaiyaensis already occurred prior to the hyperthermal event, both in open ocean and ocean margin deposits, it is proposed that these taxa are differentiated in order to avoid taxonomic and biostratigraphic ambiguities. Acarinina multicamerata n. sp. occurred exclusively during the PETM, hence this taxon represents an excellent biostrati-graphic marker of the PETM, while its common occurrence in various marine settings makes it an excellent marker of Subzone P5b or its new equivalent zone E1.