The molecular mechanisms underlying mycorrhizal symbioses, the most ubiquitous and impactful mutualistic plant-microbial interaction in nature, are largely unknown. Through genetic mapping, resequencing and molecular validation, we demonstrate that a G-type lectin receptor-like kinase (lecRLK) mediates the symbiotic interaction between Populus and the ectomycorrhizal fungus Laccaria bicolor. This finding uncovers an important molecular step in the establishment of symbiotic plant-fungal associations and provides a molecular target for engineering beneficial mycorrhizal relationships.