The hantavirus membrane fusion process is mediated by the Gc envelope glycoprotein from within endosomes. However, little is known about the specific mechanism that triggers Gc fusion activation, and its pre- and post-fusion conformations. We established cell-free in vitro systems to characterize hantavirus fusion activation. Low pH was sufficient to trigger the interaction of virus-like particles with liposomes. This interaction was dependent on a pre-fusion glycoprotein arrangement. Further, low pH induced Gc multimerization changes leading to non-reversible Gc homotrimers. These trimers were resistant to detergent, heat and protease digestion, suggesting characteristics of a stable post-fusion structure. No acid-dependent oligomerization rearrangement was detected for the trypsin-sensitive Gn envelope glycoprotein. Finally, acidification induced fusion of glycoprotein-expressing effector cells with non-susceptible CHO cells. Together, the data provide novel information on the Gc fusion trigger and its non-reversible activation involving lipid interaction, multimerization changes and membrane fusion which ultimately allow hantavirus entry into cells.