Broad-spectrum herbicide resistance (BSHR), often linked to weeds with metabolism-based herbicide resistance, poses a threat to food production. Past studies have revealed that overexpression of catalytically promiscuous enzymes explains BSHR in some weeds; however, the mechanism of BSHR expression remains poorly understood. Here, we investigated the molecular basis of high-level resistance to diclofop-methyl in BSHR late watergrass (Echinochloa phyllopogon) found in the United States, which cannot be solely explained by the overexpression of promiscuous cytochrome P450 monooxygenases CYP81A12/21. The BSHR late watergrass line rapidly produced 2 distinct hydroxylated diclofop acids, only 1 of which was the major metabolite produced by CYP81A12/21. RNA-seq and subsequent reverse transcription quantitative PCR (RT-qPCR)-based segregation screening identified the transcriptionally linked overexpression of a gene, CYP709C69, with CYP81A12/21 in the BSHR line. The gene conferred diclofop-methyl resistance in plants and produced another hydroxylated diclofop acid in yeast (Saccharomyces cerevisiae). Unlike CYP81A12/21, CYP709C69 showed no other herbicide-metabolizing function except for a presumed clomazone-activating function. The overexpression of the 3 herbicide-metabolizing genes was also identified in another BSHR late watergrass in Japan, suggesting a convergence of BSHR evolution at the molecular level. Synteny analysis of the P450 genes implied that they are located at mutually independent loci, which supports the idea that a single trans-element regulates the 3 genes. We propose that transcriptionally linked simultaneous overexpression of herbicide-metabolizing genes enhances and broadens the metabolic resistance in weeds. The convergence of the complex mechanism in BSHR late watergrass from 2 countries suggests that BSHR evolved through co-opting a conserved gene regulatory system in late watergrass.