Polymorphism is typical for most crystalline compounds, but the number of polymorphic modifications can differ significantly (up to one order). The reasons for such a diverse number of (meta)stable crystalline modifications remain unclear. Notably, different polymorphs have distinct physical properties, and their identification is of paramount importance for modern materials science. Here, the crystal structures, thermal properties and phase transformations of the hexacelsian-type BaZn2P2O8 polymorphs were characterized by in situ single crystal low- and high-temperature (in temperature range from −173 to +1000 °C) X-ray diffraction (XRD), high-temperature powder XRD and micro-Raman spectroscopy. Our studies did not confirm the previously described trigonal crystal structure of the compound under ambient conditions (correct space group C2 instead of P–3m) and showed the presence of at least two high-temperature modifications (P–3c and P–3m) previously unidentified. Additionally, we reviewed structural data of hexacelsian-type compounds and showed that most of them are characterized by an unusually high number of polymorphic modifications. Detailed X-ray diffraction studies are needed for correct identification of different polymorphs, while other methods could not detect hidden phase transitions.