Mechanical properties of polyaniline (PANI) coated self-assembled tobacco mosaic virus (TMV) nanotubes were measured by nanoindentation with an atomic force microscope (AFM) probe. The elastic modulus of PANI/TMV core-shell biocomposite tubes (outer diameter: 19 nm, inner diameter: 4 nm, PANI coating thickness: 0.5 nm) was measured from the AFM force-displacement curves. Structural characteristics of the PANI coating, TMV tube, and their interface were studied by comparing the mechanical properties of individual constituent materials. Results showed that coating with a thin layer of PANI can change the physical and chemical properties of TMV nanotubes. The deformation behavior of such core-shell biocomposite tubes is discussed in conjunction with the obtained AFM force-displacement curves, van der Waals interaction between PANI and TMV, and mechanical properties of individual constituent materials.