Largemouth bass ranavirus (LMBV) has been recognized as the causative pathogen responsible for infectious skin ulcerative syndrome in cultured largemouth bass in China. A fast and simple LMBV detection method is urgently needed. Here, a loop-mediated isothermal amplification (LAMP) assay was established for the detection of this virus using primers targeting the major capsid protein gene of LMBV. The amplification conditions were optimized; the assay was specific for the diagnosis of LMBV, as there was no cross-reactivity with other four Iridoviridae viruses (large yellow croaker iridovirus, Singapore grouper iridovirus, tiger frog virus, and soft-shelled turtle iridovirus), grass carp reovirus, white spot syndrome virus, or healthy largemouth bass. The sensitivity of the LAMP assay was found to be 8.55 × 10¹ copies/μL of LMBV DNA, which was 10-fold higher than that of the conventional PCR. Application of the LAMP assay was evaluated using 10 clinical samples, and the results indicated the reliability of the test as a rapid, field diagnostic tool for LMBV detection. Thus, the simplicity and nearly instrument-free LAMP method provides an alternative for rapid and sensitive detection of LMBV and has great potential for early diagnosis of LMBV infection in the farm.