Abstract: Winterhardiness is a complex trait that involves low temperature tolerance (LTT), vernalization sensitivity, and photoperiod sensitivity. Quantitative trait loci (QTL) for these traits were fi rst identifi ed using biparental mapping populations; candidate genes for all loci have since been identifi ed and characterized. In this research we used a set of 148 accessions consisting of advanced breeding lines from the Oregon barley (Hordeum vulgare L. subsp vulgare) breeding program and selected cultivars that were extensively phenotyped and genotyped with single nucleotide polymorphisms. Using these data for genome-wide association mapping we detected the same QTL and genes that have been systematically characterized using biparental populations over nearly two decades of intensive research. In this sample of germplasm, maximum LTT can be achieved with facultative growth habit, which can be predicted using a three-locus haplotype involving FR-H1, FR-H2, and VRN-H2. The FR-H1and FR-H2 LTT QTL explained 25% of the phenotypic variation, offering the prospect that additional gains from selection can be achieved once favorable alleles are fi xed at these loci.