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Black rot, caused by <i>Xanthomonas campestris</i> pv. <i>campestris</i> (<i>Xcc</i>), produces V-shaped chlorotic lesions on the leaves of cabbage (<i>Brassica oleracea</i> var. <i>capitata</i> L.), causing darkened veins and drastically reducing yield and quality. Of the 11 <i>Xcc</i> races identified, races 1, 4, and 6 are predominant globally. In the present study, we aimed to develop a molecular marker linked to black rot resistance against <i>Xcc</i> races 6 and 7. Crossed between black rot-resistant (‘SCNU-C-3470’) and -susceptible (‘SCNU-C-3328’) lines obtained 186 F₂ plants. Resistance to <i>Xcc</i> race 6 segregated in a 3:1 (susceptible:resistant) ratio in the F₂ population, which is consistent with a monogenic recessive trait. Nucleotide-binding site (NBS) leucine rich repeat (LRR)-encoding resistance (<i>R</i>) genes play a crucial role in plant defenses to various pathogens. The candidate <i>R</i> gene (<i>Bol031422</i>) located on chromosome C08, previously reported by our research group, was cloned and sequenced in resistant and susceptible cabbage lines. The <i>R</i> gene <i>Bol031422</i> consisted of a single exon with a 3 bp insertion/deletions (InDels), a 292 bp polymorphism (an insertion in the exon of the resistant line relative to the susceptible line) and several single nucleotide polymorphisms (SNPs). Here, we developed the InDel marker BR6-InDel to assess linkage between variation at <i>Bol031422</i> and resistance to <i>Xcc</i> races 6 and 7. This marker will help cabbage breeders develop cabbage cultivars resistant to <i>Xcc</i> races 6 and 7.