We report here the preparation of filamentous virus-like particles by the encapsulation of a linear or circular double-stranded DNA template with preassembled mushroom-shaped nanostructures having a positively charged domain. These nanostructures mimic the capsid proteins of natural filamentous viruses and are formed by self-assembly of coiled-coil peptides conjugated at opposite termini with cationic segments and poly(ethylene glycol) (PEG) chains. We found that a high molecular weight of PEG segments was critical for the formation of monodisperse and uniformly shaped filamentous complexes. It is proposed that electrostatic attachment of the nanostructures with sufficiently long PEG segments generates steric forces that increase the rigidity of the neutralized DNA template. This stiffening counterbalances the natural tendency of the DNA template to condense into toroids or buckle multiple times. The control achieved over both shape and dimensions of the particles offers a strategy to create one-dimensional supramolecular nanostructures of defined length containing nucleic acids.