The herbicidal activity and properties of a novel paddy herbicide, benzobicyclon [3-(2-chloro-4-mesylbenzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-one], were examined under flooded conditions. Benzobicyclon showed broad-spectrum activity at doses of 200–300 g a.i./ha when applied from pre-emergence to early post-emergence against annual grass, sedge, and broadleaf weeds in paddy. Benzobicyclon has excellent selectivity of transplanted rice. The most significant herbicidal symptom was bleaching. Benzobicyclon had a wide application window and controlled Scirpus juncoides up to 5 leaf stages, which is difficult to control throughout the season using other paddy herbicides. Benzobicyclon controlled S. juncoides much faster under high temperature (25°C) than under low temperature (15°C); however, the herbicidal efficacy was almost the same, regardless of the temperature, when the leaf stage of the untreated control was the same. The residual activity of benzobicyclon on S. juncoides lasted for at least 8 weeks. The herbicidal activity of benzobicyclon at doses of 200–300 g a.i./ha was not affected by the emergence depth (0–3 cm) of S. juncoides or soil types (clay loam, loam, light clay, sandy loam, and heavy clay). Moreover, benzobicyclon controlled sulfonylurea herbicide-resistant biotypes as well as wild-type weeds.