In August 2016, lettuce plants (Lactuca sativa) cultivated in Gunma Prefecture became stunted and often wilted. Dark brown to black lesions were observed on roots of diseased plants. Most small feeder roots were lost. Fungi isolated from lesions on roots were identified as Berkeleyomyces rouxiae based on their morphological and genetic characteristics. Pathogenicity of this fungus on lettuce was confirmed by inoculation and reisolation. This report is the first of black root rot caused by B. rouxiae on lettuce in Japan.