OCIMUM basilicum L. (a medicinal plant known as sweet basil) was investigated and showed disease symptoms, including mosaic mottling, leaf distortion, blistering, witches' broom, and phyllody. The plants were found to be infected by both tobacco mosaic virus (TMV) and phytoplasma. Positive TMV was indicated by serological identification using direct antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Phytoplasma infection was identified molecularly using the phytoplasma P1/P7 primer pair and the nested primer, R16F2n/R16R2, which successfully generated a DNA band at ~1200 bp. The virus was successfully transmitted mechanically to Nicotiana tabacum, Datura metal, Datura stramonium, and Phaseolus vulgaris. Light and transmission electron microscopy showed pleomorphic phytoplasmas and rod-shaped particles (300 nm) of TMV, respectively. Histopathological and cytopathological studies revealed changes in the cell behavior of the infected leaves of sweet basil due to virus and phytoplasma infection. Hypoplasia and hyperplasia were observed in the mosaic areas of the mesophyll. Alterations in the vascular tissues and degeneration of the organelles were observed in ultrathin sections of the infected mesophyll. The presence of phytoplasma cells and crystalline bodies were indicative of the mixed infection. To our knowledge, this is the first report of a mixed infection of TMV and phytoplasmas in sweet basil.