Plasmonic anisotropic nanocomposites containing in particular nanorods and nanowires are gaining increasing interest due to their remarkable physical and optical properties. In this paper, we introduce a novel effective medium theory which we call “Orientation and Shape Distributions Effective Medium Theory” (OSDEMT), to describe the optical properties of a 3D arrangement of metallic nanorods in a dielectric matrix. The effective dielectric properties of plasmonic nanocomposites containing nanorods with different shape and orientation configurations were simulated and analyzed using OSDEMT. We demonstrate that the amplitudes of the longitudinal and transverse surface plasmonic resonances depend on the orientation distribution of the nanostructures, when their positions remain unchanged. Moreover, we show that the anisotropic properties of the nanocomposites are very sensitive to the aspect ratios of the nanorods so that even spherical-centered shape distributions exhibit anisotropic properties when oriented. Finally, we confront the OSDEMT to UV–visible extinction measurements conducted on gold nanorods in a toluene suspension.