Mother-in-law’s tongue, Dracaena (Sansevieria) trifasciata S. is an ornamental houseplant of Asparagaceae family and native to tropical western Africa and southern Asia. Following a yellow withered spot symptom observed on up to one third of D. trifasciata plants at the field, a new bacterium pathogen was discovered for the first time in South of China. This pathogen was isolated from the early irregular yellow spot and its pathogenicity to the host plant was confirmed through artificial infection. Through both morphological characteristics and molecular DNA sequences using 16S rDNA and three housekeeping genes, the pathogen was identified and characterized as Paenibacillus (Bacillus) polymyxa. Obviously, our research confirmed that P. polymyxa caused the leaf spot of D. trifasciata, which indicated that the biocontrol bacteria in the inherent knowledge could also infect some plants and cause diseases.