Rice blast, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most problematic diseases for rice production, threatening global food security. Genetic resistance to some M. oryzae races can be achieved using major resistance loci containing the corresponding avirulence (AVR) genes. Weedy rice, a close relative of cultivated rice that competes with the crop, has evolved unique genetic mechanisms to resist the infections of M. oryzae; thus, weedy rice can serve as an excellent resource for blast control. In this study, we assessed disease scores of 183 F5 and F6 recombinant inbred lines (RILs) derived from a weedy rice × crop biparental mapping population and their parental lines, a Black Hull Awn weedy rice strain (PI 653413, RR14) and the aus-196 rice variety, using five blast races IB33, IB49, IG1, IE1K and ICI7 under greenhouse conditions. Except for the race IB49, both parental lines were resistant to all blast races; however, RILs showed a wide degree of variation in resistance. Genotyping-by-sequencing of the RIL population and parents generated 1498 SNPs which were used to construct a linkage map, and QTL mapping of blast resistance was performed using r/qtl. A single major blast resistance QTL on chromosome 12 was mapped to the Pi-ta/Pi39(t)/Ptr locus. Identification of the Pi-ta/Pi-t39(t)/Ptr as the key contributor to blast resistance in weedy rice provides insight into the evolution and adaptation of weedy rice and can aid in development of blast resistant rice varieties through marker-assisted selection.