Viral RNA synthesis of mononegaviruses occurs in cytoplasmic membraneless organelles called inclusion bodies (IBs). Here, we report that IBs of mumps virus (MuV), which is the causative agent of mumps and belongs to the family , displayed liquid organelle properties formed by liquid-liquid phase separation. Super-resolution microscopic analysis of MuV IBs demonstrated that nucleocapsid and phospho (P)-proteins formed a cage-like structure and that the viral polymerase adopted a reticular pattern and colocalized with viral RNAs. In addition, we characterized host RNAs localized in MuV IBs by a spatial transcriptome analysis, and found that RNAs containing G-quadruplex motif sequences (G4-RNAs) were concentrated. An in vitro phase separation assay showed that the G4-RNAs interacted with the P protein and enhanced condensation in P droplets. Together, our data show that MuV generates IBs with a characteristic cage-like structure and host G4-RNAs play an important role in forming MuV IBs.