Levitated nanospheres are a promising platform for realizing large superpositions of massive objects. Interfacing such levitated nanoparticles with nanophotonic structures, e.g., chips and waveguides, can allow one to control and probe their quantum states efficiently. In this paper, we study the center-of-mass decoherence of a polarizable nanoparticle in the presence of a planar metallic medium due to thermal fluctuations of the electromagnetic (EM) field.