Nine samples of diseased Malvastrum coromandelianum plants collected from the fields in Yunnan province of China were found to be infected with Malvastrum yellow vein Yunnan virus (MYVYNV), when tested by PCR using specific primers. The results of PCR and Southern blot analysis showed that only 4 samples out of 9 were associated with the satellite DNAb molecules. Their sequence analysis indicated that DNAb molecules share 97.8%-99.4% of nucleotide sequence identities with DNAb associated with MYVYNV isolate Y160 and less than 82.0% with those associated with other begomoviruses. Two infectious clones of MYVYNV (isolates Y160 and Y277) produced yellow vein, vein thickening and upward leaf curl symptoms in Nicotiana benthamiana plants. In the presence of its cognate DNAb, the symptoms changed to downward leaf curl and crinkle. Southern blot analysis showed that DNAb could increase accumulation of its cognate virus in the infected N. benthamiana plants. The above results indicated that MYVYNV is a monopartite begomovirus and its association with DNAb is not necessary for the infection of plants, but is able to intensify symptoms specific for the disease.