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Drought stress poses a serious threat to <i>Pinus massoniana</i> seedling growth in southern China. <i>Trichoderma</i> species, as beneficial microorganisms, have been widely used in agriculture to enhance plant growth and drought tolerance, but the interaction mechanisms remain unclear. To investigate the effect of drought-resistant <i>Trichoderma longibrachiatum</i> inoculation on <i>P. massoniana</i> growth under drought stress, the plant physiological indicators and rhizosphere microbiome diversity were measured to identify <i>Trichoderma</i>-activated mechanisms. <i>Trichoderma longibrachiatum</i> inoculation significantly promoted <i>P. massoniana</i> growth under drought treatment, and enhanced nitrogen, phosphorus, and potassium absorption compared with those of non-inoculated seedlings. <i>Trichoderma longibrachiatum</i> treatment alleviated the damage to cell membranes and needle tissue structure, and significantly increased antioxidant enzyme activities, osmotic substance contents, and photosynthesis in <i>P. massoniana</i> in response to drought stress. Soil nutrient contents, activities of sucrase, phosphatase, and urease as well as the relative abundances of the dominant genera <i>Burkholderia</i>, <i>Rhodanobacter</i>, and <i>Trichoderma</i> were elevated in the rhizosphere soil of <i>P. massoniana</i> inoculated with <i>T. longibrachiatum</i> under drought stress. A network analysis showed that certain crucial dominant taxa driven by <i>T. longibrachiatum</i> inoculation, including <i>Penicillium</i>, <i>Trichoderma</i>, <i>Simplicillium</i>, <i>Saitozyma</i>, <i>Burkholderia</i>, <i>Bradyrhizobium</i>, <i>Sinomonas</i>, and <i>Mycobacterium</i>, had more correlations with other microorganisms in the soil. <i>Trichoderma longibrachiatum</i> enhanced <i>P. massoniana</i> seedling growth under drought stress by regulating physiological responses and soil microbial community.