Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is not only a disease devastating rice production worldwide, but also an ideal model system for the study of the interaction between plants and their bacterial pathogens. The rice near‐isogenic line (NIL) CBB23, derived from a cross between a wild rice Oryza rufipogon accession (RBB16) and a susceptible indica rice variety (Jingang 30), is highly resistant to all field Xoo strains tested so far. Although the BB resistance of CBB23 has been widely used in rice breeding programmes, the mechanism of its extremely broad‐spectrum resistance remains unknown. Here, we report the molecular cloning of an avirulence gene, designated as avrXa23, from Xoo strain PXO99A. We validate that AvrXa23, a novel transcription activator‐like effector, specifically triggers the broad‐spectrum BB resistance in CBB23. The prevalence of avrXa23 in all 38 Xoo strains surveyed may explain the broad‐spectrum feature of BB resistance in CBB23. The results will significantly facilitate the molecular cloning of the corresponding resistance (R) gene in the host, and provide new insights into our understanding of the molecular mechanism for broad‐spectrum disease resistance in plants.