The Japanese encephalitis virus (JEV) is a zoonotic pathogen that affects the nervous systems of humans, pigs, and horses. It has been classified into five genotypes (G1-G5) based on molecular analysis of the pre-membrane or envelope gene. In the Republic of Korea, the predominant JEV genotype has recently shifted from G3 to G1 and G5, highlighting the need for a rapid and accurate diagnostic method. In this study, we designed specific common and differential primer sets for JEV G1, G3, and G5 to detect the JEV gene. Four specific primer sets for JEV G1, G3, and G5 were used to selectively amplify the target gene. The detection limits of the common primer set for JEV G1, G3, and G5 were 100, 0.1, and 10 TCID50/reaction, respectively. The detection limits of the three differential primer sets were 1, 0.1, and 1 TCID50/reaction, respectively. No cross-reactivity was observed with non-JEV reference viruses. We successfully developed a multiplex reverse transcription polymerase chain reaction (RT-PCR) assay to distinguish the three JEV genotypes. Our multiplex RT-PCR assay is highly sensitive and specific, providing a reliable tool for confirming JEV infection in suspected samples. Additionally, our assay can be applied to suspected mosquito samples and commercial veterinary biological products.