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Background and Aim: Various methods can detect foot-and-mouth disease (FMD) in cows, but they necessitate resources, time, costs, laboratory facilities, and specific clinical specimen submission, often leading to FMD virus (FMDV) diagnosis delays. The 2022 FMD outbreak in East Java, Indonesia, highlighted the need for an easy, inexpensive, rapid, and accurate detection approach. This study aims to devise a one-step reverse transcriptase loop-mediated isothermal amplification (RT-LAMP) technique and phylogenetic analysis to detect the serotype O FMDV outbreak in East Java. Materials and Methods: Swab samples were collected from the foot vesicles, nasal secretions, and saliva of five suspected FMDV-infected cows in East Java between June and July 2022. The RT-LAMP design used hydroxy naphthol blue dye or SYBR Green I dye, with confirmatory analysis through reverse transcriptase polymerase chain reaction (RT-PCR) targeting 249 base pairs. PCR products underwent purification, sequencing, and nucleotide alignment, followed by phylogenetic analysis. Results: The RT-LAMP method using hydroxy naphthol blue dye displayed a positive reaction through a color shift from purple to blue in the tube. Naked-eye observation in standard light or ultraviolet (UV) light at 365 nm, with SYBR Green I stain, also revealed color change. Specifically, using SYBR Green I dye, UV light at 365 nm revealed a color shift from yellow to green, signifying a positive reaction. Nucleotide alignment revealed mutations and deletion at the 15th sequence in the JT-INDO-K3 isolate from the East Java FMDV outbreak. Despite differing branches, the phylogenetic tree placed it in the same cluster as serotype O FMDV from Malaysia and Mongolia. Conclusion: JT-INDO-K3 exhibited distinctions from Indonesian serotype O FMDV isolates and those documented in GenBank. Then, the RT-LAMP method used in this study has a detection limit 10 times higher latter than the conventional RT-PCR limit, without any cross-reactivity among strains.