Two pairs of specific primers were designed based on the grass carp reovirus (GCRV) VP6 coding gene sequence. Using GCRV genomic total RNA as a template, we developed a reverse transcription-loop mediated isothermal amplification (RT-LAMP) assay. After optimizing the reaction conditions, we were able to successfully amplify the targeted GCRV VP6 coding gene at 63 in 1 h. Analysis of the amplified products by agarose electrophoresis revealed that the band pattern resembled that of the ladder diagram in a gel. After adding SYBR Green I fluorescent dye to the reaction system, the green positive amplification could be viewed by eye. Our assay was highly sensitive to grass carp reovirus with a lower detection limit of 33 pg, which was 10-fold higher than that of traditional RT-PCR for GCRV. Moreover, the assay was specific for the detection of GCRV and was not susceptible to cross reaction with other viruses, including Channel catfish reovirus (CCRV), Spring viremia of carp virus (SVCV), Koi herpesvirus (KHV), and giant salamander iridovirus (GSIV). In conclusion, the RT-LAMP assay is convenient, rapid, sensitive, and specific for GCRV detection. Our assay provides a novel approach for the detection of GCRV and the diagnosis of grass carp hemorrhage.