Most paramyxovirus fusion proteins require coexpression of and activation by a homotypic attachment protein, hemagglutinin-neuraminidase (HN), to promote membrane fusion. However, the molecular mechanism of the activation remains unknown. We previously showed that the incorporation of a monohistidylated lipid into F-virosome (Sendai viral envelope containing only fusion protein) enhanced its fusion to hepatocytes, suggesting that the histidine residue in the lipid accelerated membrane fusion. Therefore, we explored whether a histidine moiety in HN could similarly direct activation of the fusion protein. In membrane fusion assays, the histidine substitution mutants of HN (H247A of Sendai virus and H245A of human parainfluenza virus 3) had impaired membrane fusion promotion activity without significant changes in other biological activities. Synthetic 30-mer peptides corresponding to regions of the two HN proteins containing these histidine residues rescued the fusion promoting activity of the mutants, whereas peptides with histidine residues substituted by alanine did not. These histidine-containing peptides also activated F-virosome fusion with hepatocytes both in the presence and in the absence of mutant HN in the virosome. We provide evidence that the HN-mimicking peptides promote membrane fusion, revealing a specific histidine "switch" in HN that triggers fusion.