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Rhizoctonia root rot is a disease that can affect soybean leading to plant death usually in bare patches. Frequently, Rhizoctonia solani has been reported as the main causal agent of the disease. However, due to the great genetic diversity of the genus Rhizoctonia, there are other species that could also attack soybean plants. Thus, the objective of this study was to determine the pathogenicity and characterize an isolate of binucleate Rhizoctonia obtained from a soybean plant with symptoms of root rot cultivated in soil from southern Brazil. The isolate was identified based on the morphology and phylogenetic analysis of the internal transcribed spacer region (ITS) of the rDNA. Pathogenicity tests were then performed on seeds and roots of soybean and the optimum temperature for mycelial growth was determined. The isolate was identified as binucleate Rhizoctonia AG‐Fb and it was pathogenic to soybean seeds and roots. In the pathogenicity assay in soybean seed, the isolate reduced seed germination by 20% and caused seed rot index of 69.3. In the pathogenicity assay in soybean root, the inoculation of binucleate Rhizoctonia AG‐Fb caused 67.3 root rot severity index, and reduced the emergence speed index by 19%, seedling height by 31%, root dry mass by 25% and root volume by 43% in soybean seedlings. The optimum temperature for the mycelial growth of the isolate was 26°C. To the best of our knowledge, this is the first report of binucleate Rhizoctonia AG‐Fb associated with soybean root rot in Brazil. [ABSTRACT FROM AUTHOR]