Enveloped viruses utilize surface glycoproteins to bind and fuse with a target cell membrane. The zoonotic Hendra virus (HeV)， a member of the family， utilizes the attachment protein (G) and the fusion protein (F) to perform these critical functions. Upon triggering， the trimeric F protein undergoes a large， irreversible conformation change to drive membrane fusion. Previously， we have shown that the transmembrane domain (TM) of the F protein， separate from the rest of the protein， is present in a monomer-trimer equilibrium. This TM-TM association contributes to the stability of the pre-fusion form of the protein， supporting a role for TM-TM interactions in control of F protein conformational changes. To determine the impact of disrupting TM-TM interactions， constructs expressing the HeV F TM with limited flanking sequences were synthesized. Co-expression of these constructs with HeV F resulted in dramatic reductions in the stability of F protein expression and fusion activity. In contrast， no effects were observed when the HeV F TM constructs were co-expressed with the non-homologous parainfluenza virus 5 (PIV5) fusion protein， indicating a requirement for specific interactions. To further examine this， a TM peptide homologous to the PIV5 F TM domain was synthesized. Addition of the peptide prior to infection inhibited viral infection with PIV5， but did not significantly affect infection of human metapneumovirus， a related virus. These results indicate that targeted disruption of TM-TM interactions significantly impact viral fusion protein stability and function， presenting these interactions as a novel target for antiviral development. Enveloped viruses require virus-cell membrane fusion to release the viral genome and replicate. The viral fusion protein triggers from the pre- to post-fusion conformation， an essentially irreversible change， to drive membrane fusion. We found that small proteins containing the TM and a limited flanking region homologous to the fusion protein of the zoonotic Hendra virus reduced protein expression and fusion activity. The introduction of exogenous TM peptides may displace a TM domain， disrupting native TM-TM interactions and globally destabilizing the fusion protein. Supporting this hypothesis， we showed that a sequence-specific transmembrane peptide dramatically reduced viral infection in another enveloped virus model， suggesting a broader inhibitory mechanism. Viral fusion protein TM-TM interactions are important for protein function， and disruption of these interactions dramatically reduces protein stability.