Listeria monocytogenes (L. monocytogenes) causes listeriosis in people and animals. After infection, the main symptoms of listeriosis are sepsis, meningitis and mononucleosis. For this purpose, visual detection of L. monocytogenes was developed with unmodified gold nanoparticles. Species-specific probes, in the presence of PCR target products of L. monocytogenes, cause the gold nanoparticles to aggregate irreversibly, producing a red to purple colorimetric change. As little as 1.304 fg µl⁻¹ of DNA of L. monocytogenes were thus detected visually, by the naked eye, without the need for any sophisticated, expensive instrumentation and biochemical reagents, which shows that the method has approximately seventy-fold higher sensitivity than the conventional PCR gel electrophoresis method. The results indicate that this assay is highly species-specific, simple, low-cost, and visual for easy detection of L. monocytogenes.