In July and September 2020, a severe outbreak of stem rot was observed on bonnet bellflower (Codonopsis lanceolata) plants in a farm located in Chuncheon, Gangwon Province, Korea. The symptoms initially appeared on the stem at or above the soil line. Later, the infected stem completely rotted and blighted. The incidence of diseased plants in the field was 2‒30%. Ten isolates of Rhizoctonia sp. were obtained from the stem lesions of diseased plants. All isolates were identified as Rhizoctonia solani AG-4 based on the morphological characteristics and anastomosis test. Three isolates of R. solani AG-4 were tested for pathogenicity on bonnet bellflower plants through artificial inoculation. All tested isolates induced stem rot symptoms on the inoculated plants. The symptoms were similar to those observed in plants from the farm. This is the first report of R. solani AG-4 causing stem rot in bonnet bellflower.