Late blight continues to negatively impact potato growers due to yield losses and expensive control measures. Utilizing resistance genes provides a way to control the disease in a more environmentally safe and economical manner, and the RB (aka blb1) gene offers durable resistance to most strains of Phytophthora infestans. Using two segregating tetraploid populations, a previously published SCAR (sequence-characterized amplified region) marker was compared against results from a detached leaf assay with P. infestans genotype US-23. Using haplotype analysis, the discrepancies between the two methods were resolved in favor of the SCAR marker as the more accurate predictor of the presence of RB. RB-positive individuals from this population were used to design a KASP marker, which has advantages for high-throughput screening compared to SCAR markers. By aligning whole-genome sequencing data for RB-positive and RB-negative individuals to the reference sequence for RB, a haplotype-specific SNP was discovered in the 5 untranslated region (UTR) and submitted to LGC for KASP marker design. The KASP assay showed perfect agreement with the SCAR marker in a diverse group of tetraploid and diploid potato clones.