Ectomycorrhizal fungi form symbiotic relationships with the roots of most tree species and support increased forest health and productivity. The ectomycorrhizal fungus Laccaria laccata was successfully transformed through Agrobacterium tumefaciens-mediated transformation (ATMT). We observed a transformation efficiency of 80 %, with 100 % of the assessed transformants exhibiting mitotic stability after five passages in culture medium without the selective agent. The amplification of the 626-bp DNA fragment corresponding to the hygromycin B phosphotransferase (hph) gene and a Southern blotting analysis showed that the assessed transformants had integrated the T-DNA into the genome, confirming the success of the ATMT procedure. Single T-DNA integrations were detected in five of the eight transformants assessed by Southern blotting, and the hybridization pattern confirmed that the integration occurred randomly in the genome of the fungus. The development of an efficient ATMT system that allows the integration of single T-DNA copies into the L. laccata genome makes functional genetic studies in this ectomycorrhizal fungus feasible.