The phytohormone ethylene regulates plant growth, development, and responses to both biotic and abiotic stresses. Ethylene also negatively regulates rhizobial symbiosis in legumes, although the intrinsic ethylene signaling components in legumes are still largely unclear. We report a novel ethylene insensitive mutant named Ljetr1 from the model legume Lotus japonicus. Ljetr1 showed growth tolerance to high concentrations of 1-amino-cyclopropane-carboxylic acid, the biosynthetic precursor of ethylene. Petal senescence and abscission were delayed and number of nodules was slightly increased compared to wild-type. Mapping analysis and genome sequencing showed that Ljetr1 bears a mutation in the ethylene-binding domain of the Arabidopsis ETR1 homolog. Our results suggest that the Lotus intrinsic ethylene receptor LjETR1 regulates the ethylene signaling pathway in both non-symbiotic and legume-specific symbiotic responses.