During the Miocene Climatic Optimum, a global long-term warm interval, European mid-latitude regions experienced a subtropical palaeoclimate. In particular, areas in eastern Germany were part of a vegetational zone with evergreen broadleaved forests, characterized by subtropical taxa. Regional palaeofloristic concepts denominated this palaeovegetation Younger Mastixioideae Flora sensu Mai (1964). Type assemblage is the late Early Miocene flora of Wiesa. Here, we reevaluate its floristic composition with respect to nearest living relatives of fossil-taxa, and introduce the new approach Phytogeographic Reference Region Assessment (PRRA) to ascertain the area of most similar extant vegetation for the Wiesa assemblage. The southern belt of SE Asian subtropical evergreen broadleaved forest and its transition to tropical mountain evergreen broadleaved forest in SW China represent the most similar extant vegetation. The Wiesa assemblage is compared to two diverse plant macroassemblages from the late Oligocene and the Late Miocene, respectively, coming from the same region and palaeoenvironmental setting. It is demonstrated that diversity and abundances of subtropical taxa markedly increased towards the Early Miocene, and specific climate-sensitive taxa occurred. The regional palaeoclimate was subtropical-humid (Köppen-Trewartha type Cf), with a growing season eleven months long. The late Oligocene and Late Miocene climates were mainly distinct in nine-month growing season lengths and cooler winters (1–3 °C).