In recent years, increasing temperature and water deficiency have caused restrictions on poplar cultivation. Ectomycorrhizal fungi (ECM) play an effective role in increasing plant tolerance against water stress, salinity, and the defense of plants against pests and diseases. In this study, isolation, morphological and molecular identification of ectomycorrhizal fungi of three poplar habitats in Guilan province (Shafaroud, Gisoom and Safra-basteh sites) in Iran was carried out. Based on morphological and molecular data, the ectomycorrhizal fungus Paxillus involutus was identified as poplar ectomycorrhizae. Fungal mycelium was used to inoculate seedlings of Populus alba L. in the greenhouse condition, and the effect of the ectomycorrhizal fungus on the plant growth parameters was measured in water stress and irrigation. The results showed that establishing a symbiotic relationship between poplar seedlings and ectomycorrhizal fungus P. involutus was successful. Moreover, inoculation of the symbiotic fungus positively affected improving the growth characteristics of the inoculated plants. Thus, symbiotic seedlings with ectomycorrhizal fungus were significantly different from the non-mycorrhizal seedlings in plant growth variables including root fresh and dry weight, leaf fresh weight, stem fresh and dry weight, leaf area, and stem height (p<0.01). Also, the use of symbiotic fungus reduced the negative effects of water deficit stress on poplar seedlings and increased the tolerance in the mycorrhizal plants. Based on these findings, a coordinated plant-fungal system plays an effective role in improving the performance of poplar plants under water-stress conditions.