Many phleboviruses (family Bunyaviridae) are emerging, medically important viruses. These viruses enter target cells by endocytosis and low pH dependent membrane fusion in late endosomes. However, the necessary and sufficient factors for fusion have not been fully characterized. We have studied the minimal fusion requirements of a prototypic phlebovirus, Uukuniemi virus, in an in vitro virus-liposome assay. We show that efficient lipid mixing between viral and liposome membranes requires close-to-physiological temperatures and phospholipids with negatively charged headgroups, such as the late endosomal phospholipid bis(monoacylglycero)phosphate (BMP). We further demonstrate that BMP increases Uukuniemi virus fusion beyond the lipid mixing stage. By using electron cryo-tomography of viral particles in presence or in absence of liposomes, we observed that the conformation of phlebovirus glycoprotein capsomers changes from the native conformation towards a more elongated conformation at fusion-permissive pH. Our results suggest a rationale for phlebovirus entry in late endosomes.