Dendrobium smillieae is one of the popular orchids in Taiwan. This report described a new potyvirus tentatively named Dendrobium chlorotic mosaic virus (DeCMV) causing chlorotic and mosaic symptoms in D. smillieae. ELISA tests using six antisera against orchid-infecting viruses revealed that only a monoclonal antiserum against the potyvirus group reacted positively with crude saps prepared from a symptomatic dendrobium orchid. Potyvirus-like, flexuous filamentous particles were observed under an electron microscope, measuring approximately 700-800 nm in length and 11-12 nm in diameter. Sequence analyses revealed that DeCMV coat protein gene shared 59.6-66.0% nucleotide sequence identity and 57.6-66.0% amino acid sequence identity, whereas the DeCMV complete genome shared 54.1-57.3% nucleotide sequence identity and 43.7-49.5% amino acid sequence identity with those other known potyviruses. Thus, DeCMV can be considered as a new potyvirus. The whole DeCMV genome contains 10,041 nucleotides (MK241979) and encodes a polyprotein which is predicted to produce ten proteins by proteolytic cleavage. In pathogenicity test, results of inoculation assays demonstrated that DeCMV can be transmitted to dendrobium orchids by grafting and mechanical inoculation, as verified by ELISA using the DeCMV polyclonal antiserum and by RT-PCR using the CP gene-specific primers. The inoculated orchids developed the similar chlorotic and mosaic symptoms. In conclusion, DeCMV is a novel orchid-infecting potyvirus and this is the first report of a new potyvirus that infects dendrobium orchids in Taiwan.