Key message <p>BrRLP1 positively regulates the resistance to downy mildew in <i>Brassica rapa</i> by interacting with the monodehydroascorbate reductase BrMDAR1.</p> Abstract <p>Downy mildew is a devastating disease that severely affects the yield and quality in <i>Brassica rapa</i>. Receptor-like protein (RLP) is important for plants disease-resistant response. Here, a new downy mildew resistance gene, <i>BrRLP1</i>, was identified in <i>Brassica rapa</i> through GWAS analysis and QTL mapping. <i>BrRLP1</i> encodes a membrane-localized receptor-like protein, and its expression level showed significant differences in the resistant and susceptible materials after inoculation with downy mildew. Transient expression and transgenic functional verification revealed that <i>BrRLP1</i> is a positive regulator for the downy mildew resistance. All the <i>BrRLP1</i>^<i>R</i> overexpressed plants exhibited a high-resistance phenotype to downy mildew after inoculation. Haplotype analysis revealed that the SNP309 in the LRR domain of <i>BrRLP1</i> is a key functional site for the resistance difference to downy mildew. Y2H and LCI assays showed that BrRLP1 can interact with the monodehydroascorbate reductase BrMDAR1, which is involved in the ascorbic acid metabolic pathway. Our results revealed the function of BrRLP1 in regulation of downy mildew resistance by interacting with BrMDAR1, which provides new insight into the molecular mechanism underlying disease resistance immune response in <i>Brassica rapa</i>.</p>