We report on a new adult plant resistance (APR) gene Rph23 conferring resistance to leaf rust in barley. The gene was identified and characterized from a doubled haploid population derived from an intercross between the Australian barley varieties Yerong (Y) and Franklin (F). Genetic analysis of adult plant field leaf rust scores of the Y/F population collected over three successive years indicated involvement of two highly additive genes controlling APR, one of which was named Rph23. The gene was mapped to chromosome 7HS positioned at a genetic distance 36.6 cM. Rph23 is closely linked to marker Ebmac0603, which is flanked by markers bPb-8660 and bPb-9601 with linkage distances of 0.8 and 5.1 cM, respectively. A PCR-based marker was optimized for marker-assisted selection of Rph23, and on the basis of this marker, the gene was postulated as being common in Australian and global barley germplasm. Pedigree and molecular marker analyses indicated that the six-rowed black Russian landrace ‘LV-Taganrog’ is the likely origin of Rph23.