We report a confocal interferometric mid-infrared photothermal (MIP) microscope for ultra-sensitive and spatially resolved chemical imaging of individual viruses. The interferometric scattering principle is applied to detect the very weak photothermal signal induced by infrared absorption of chemical bonds. Spectroscopic MIP detection of single vesicular stomatitis viruses (VSVs) and poxviruses is demonstrated. The single virus spectra show high consistency within the same virus type. The dominant spectral peaks are contributed by the amide I and amide II vibrations attributed to the viral proteins. The ratio of these two peaks is significantly different between VSVs and poxviruses, highlighting the potential of using interferometric MIP microscopy for label-free differentiation of viral particles. This all-optical chemical imaging method opens a new way for spectroscopic detection of biological nanoparticles in a label-free manner and may facilitate in predicting and controlling the outbreaks of emerging virus strains.