It was shown previously that the matrix (M), phosphoprotein (P), and fusion (F) proteins of respiratory syncytial virus (RSV) are sufficient to produce viruslike particles (VLPs) that resemble the RSV infection-induced virions. However, the exact mechanism and interactions among the three proteins are not known. This work examines the interaction between P and M during RSV assembly and budding. We show that M interacts with P in the absence of other viral proteins in cells by using a split Nano luciferase assay. By using recombinant proteins, we demonstrate a direct interaction between M and P. By using nuclear magnetic resonance (NMR), we identify three novel M interaction sites on P, namely, site I in the a N2 region, site II in the 115 to 125 region, and the oligomerization domain (OD). We show that the OD, and likely the tetrameric structural organization of P, is required for virus-like filament formation and VLP release. Although sites I and II are not required for VLP formation, they appear to modulate P levels in RSV VLPs.