Columnaris disease has recently emerged as a concern for black carp (Mylopharyngodon piceus), an economically important aquaculture species in northern Vietnam. This study investigated the disease outbreaks from five affected farms. A total of 47 naturally diseased fish were sampled for bacterial isolation and identification. Five representative isolates resembling Flavobacterium were selected for phenotypic and genetic characterization. Through a combination of biochemical tests, PCR assays, and 16 S rRNA sequencing, these bacterial isolates were identified as Flavobacterium columnare. Experimental challenge tests on black carp revealed a median lethal dose (LD