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The development of durable resistance to broomrape (Orobanche cumana Wallr.) in sunflower (Helianthus annuus L.) requires detailed characterization of the genetic and physiological bases of resistance. The objective of the present study was to map the resistance gene accurately, and to characterize the mechanism of resistance to broomrape observed in a sunflower inbred line (PHSC1102). PHSC1102, which was consistently resistant against race F and race G populations of broomrape, was crossed with PHSC1201, which was susceptible to races F and G. A mapping population of 150 F2 genotypes was phenotyped by evaluating F2:3 families for resistance to broomrape races FGV and GTK. The use of single nucleotide polymorphism (SNP) markers mapped the OrSII gene to Linkage Group 4 (LG4) of the sunflower genome. Mini‐rhizotron and histological studies of the resistant line revealed that the attachment of broomrape to host roots was similar in both the resistant and susceptible lines and that the resistance was observed at a late stage (i.e., after tubercle development). Interestingly, the resistance of the PHSC1102 line was associated with the production of phenolic compounds, which were hypothesized to restrict the parasite's growth. This research provides novel and valuable information about the host–parasite interactions between sunflower and broomrape.