The intermetallic compound Cu6Sn5 is a significant microstructural feature of many electronic devices where it is present at the solder– substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu6Sn5 are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η′, η6 and η8). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu6Sn5 using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic–hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η8-Cu5Sn4 unit cells plus one η′-Cu6Sn5 unit cell. It has been labeled as η4+1 and has cell parameters of a = 92.241 Å, b = 7.311 Å, c = 9.880 Å and β = 118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.