Early Middle Pleistocene palaeolacustrine sediments of the Ferdynandów site (E Poland), serving as a stratotype for the Ferdynandovian interglacial, were subjected to a new drilling in 2011. The obtained profile, covering the late Sanian 1 glaciation, two interglacial successions (Ferdynandovian 1 and 2), the cold Ferdynandovian 1/2 interval, and the early Sanian 2 glaciation, permitted a high-resolution plant macroremains analysis, correlated with the results of a palynological examination. In detailed studies of plant macroremains from the profile, new taxa were discovered for the Polish Pleistocene flora: specifically, species that are extinct or not found nowadays in Poland. On the basis of the taxonomic diversity and occurrence of bioindicators, the division into warm and cold units of the Ferdynandovian interglacial as well as units associated with the Sanian 1 and 2 glaciations, already applied in pollen studies, were confirmed, and the periods were described in terms of plant macroremains. The Ferdynandovian succession is correlated with the Cromerian Complex (Cromerian III and IV) in the Early Middle Pleistocene of Western Europe, as well as with Marine Isotope Stages (MIS) 13–15. New geological and palaeobotanical data enabled a description of the palaeogeographic context and conditions of functioning, as well as the evolution of the interglacial lake at the stratotype site. The outcome of plant macroremains analysis is presented and interpreted in comparison with corresponding results previously obtained for the Ferdynandów B profile, studied by Janczyk-Kopikowa (1975). Particular consideration is given to similarities between the succession of Ferdynandów 2011 and the profiles of Ferdynandów B and the nearby drilled Łuków 3A. Study of the macrofossil flora of Ferdynandów 2011 revealed the presence of species absent in the archival Ferdynandów B. Taxa not formerly recorded in early Middle Pleistocene floras, such as Brasenia borysthenica, Aldrovanda borysthenica, Pilularia borysthenica, Caulinia goretskyi, and Potamogeton saryanensis, were identified. These species and other taxa of stratigraphic or climatic significance provided the basis for a more detailed description of the Ferdynandovian flora, afterwards compared with other stratigraphically corresponding Polish and European early Middle Pleistocene floras. The composition of characteristic taxa also was used for comparisons between two bimodal interglacial sequences, namely the Ferdynandovian (correlated with MIS 13–15) and Augustovian (correlated with MIS 19–21), and the Domuratovian succession (correlated with MIS 17).