Xanthomonas oryzae pv oryzae (Xoo) causes bacterial leaf blight, a devastating disease in rice crops worldwide. Early diagnosis is critical to limit economic losses. We report a rapid, cost-effective colorimetric biosensor using unmodified gold nanoparticles (AuNPs) and a novel Xoo-specific DNA probe for visual detection of the pathogen. The detection strategy exploits the surface plasmonic property of AuNP solutions that shifts their color to purple/blue upon probe–target binding-induced nanoparticle aggregation. AuNPs were functionalized with probes complementary to a conserved Xoo genomic sequence. Genomic DNA was extracted from pure Xoo cultures and infected rice leaf samples. Aggregation was evaluated after mixing probe-functionalized AuNPs with DNA samples. The color change from red to purple was visually observed within 5 minutes of mixing probe AuNPs with as little as 5 ng/μL of purified Xoo DNA and 0.5 g of infected leaf tissue. No color change occurred using DNA from healthy plants or other pathogens, indicating a high specificity. This assay offers a simple, rapid, and field-deployable colorimetric method for visual detection of Xoo with a high sensitivity and specificity. The unmodified AuNPs and specific probe provide a cost-effective alternative to PCR-based methods. This biosensor has immense potential for on-site diagnosis and management of bacterial leaf blight in rice plants.