Waitea circinata var. circinata was identified as the causal agent of a new disease of annual bluegrass (Poa annua) in the United States. This pathogen is also known to cause brown ring patch on creeping bentgrass (Agrostis stolonifera) in Japan, but it had not been reported on any turf species outside of Japan. Symptoms on annual bluegrass caused by this fungus included regular to irregular yellow rings several centimeters to 1 m in diameter, typically at maximum daytime temperatures of 15 to 35°C. A total of 26 isolates were collected from diseased annual bluegrass. Twenty-two of these isolates were multinucleate, grew optimally at 25 to 30°C, and in culture formed irregular sclerotia approximately 2 to 5 mm in size that were white to orange and remained orange or turned brown to dark brown over a 28-day period. The remaining four isolates were characterized as being W. circinata var. zeae (Rhizoctonia zeae), which is a known pathogen of annual bluegrass in the United States. Sequence analysis of the internal transcribed spacer (ITS) regions of rDNA on a subset of isolates confirmed the identifications of W. circinata var. circinata (n = 8) and W. circinata var. zeae (n = 1) based on deposited sequences in GenBank. The identity of the remaining 14 isolates suspected to be W. circinata var. circinata was confirmed by HapII digestion of the amplified rDNA ITS region. Pathogenicity of four W. circinata var. circinata isolates was confirmed on both annual bluegrass and creeping bentgrass. This study is the first morphological and molecular confirmation of the presence of W. circinata var. circinata as a pathogen of turfgrass in the United States.