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Smallflower umbrella sedge (<i>Cyperus difformis</i> L.) is an invasive weed, and infestations of <i>C</i>. <i>difformis</i> are increasing in rice (<i>Oryza sativa</i> L.) fields in China. Bensulfuron-methyl is a widely used sulfonylurea herbicide that inhibits the acetolactate synthase (ALS) enzyme and has been used in recent years for effectively controlling annual weeds in the Cyperaceae family. In this study, a suspected resistant population of <i>C. difformis</i> (BBHY1) was collected from a rice field in Huaiyuan County, Anhui Province, China, that survived treatment with bensulfuron-methyl at the field-recommended rate (FRR). Single-dose tests and whole-plant bioassays confirmed that the BBHY1 population was resistant to bensulfuron-methyl and had evolved a high level of resistance, with a resistance index (RI) of 12.87. Sequencing of the <i>ALS</i> gene revealed a CCT to CAT point mutation at codon 197, which caused a P-to-H substitution in the resistant plants. Analysis of the relative expression of <i>ALS</i> revealed no significant differences between the resistant and susceptible populations. Inhibiting the activity of cytochrome P450s (P450s) or glutathione S-transferases (GSTs) had no significant effect on bensulfuron-methyl resistance. The BBHY1 population exhibited cross-resistance to pyrazosulfuron-ethyl, penoxsulam, and bispyribac-sodium, with RIs ranging from 5.48 to 20.63, but remained susceptible to MCPA sodium, florpyrauxifen-benzyl, and bentazon, with RIs of <1.00. These herbicides could be potentially used as alternatives for controlling resistant populations and managing herbicide resistance in other aggressive weeds in rice fields.