Barley is a near-nonhost to numerous heterologous (nonadapted) rust pathogens because a small proportion of genotypes are somewhat susceptible. We assessed 66 barley accessions and three mapping populations (Vada × SusPtrit, Cebada Capa × SusPtrit, and SusPtrit × Golden Promise) for response to three Swedish oat stem rust (Puccinia graminis f. sp. avenae) fungal isolates and determined that barley is a near-nonhost to P. graminis f. sp. avenae and that resistance was polygenically inherited. The parental genotypes Vada and Golden Promise were immune to all three isolates, whereas Cebada Capa was immune to two isolates and moderately resistant to the third. Phenotypic data from the Vada × SusPtrit mapping population and the barley accessions tested also demonstrated isolate-specific resistance. In particular, the SusPtrit parent and several other accessions allowed sporulation by isolate Ingeberga but were resistant to isolate Evertsholm. Nine chromosomal regions carried quantitative trait loci (QTL) (Rpgaq1 to Rpgaq9) of varying effect, most of which colocated to previously identified QTL for resistance to other heterologous rust pathogens. Rpgaq1 on chromosome 1H (Vada and Golden Promise) was effective toward all isolates tested. Microscopic examination indicated that resistance was prehaustorial in Vada whereas, in SusPtrit, both pre- and posthaustorial mechanisms play a role.