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<i>Serendipita indica</i> is a very promising root-associated endophytic fungus that is widely used on various plants; however, whether it affects the growth and physiological activity of an oilseed crop (<i>Camellia oleifera</i>) under field conditions remains unclear. In this study, we analyzed the effects of <i>S. indica</i> inoculation on root colonization rate, growth rate, photosynthetic parameters, mineral element concentrations and related gene expression, and sugar concentrations and expression of their transporter genes in four-year-old <i>C. oleifera</i> trees in the field. The results showed that the root colonization rate of <i>C. oleifera</i> increased from 3.37% to 9.42% following being inoculated with <i>S. indica</i>. Inoculation with <i>S. indica</i> significantly increased the plant height (46.81%), net photosynthetic rate (69.16%), nitrogen balance index (14.44%), chlorophyll index (21.08%), leaf K (7.4%), leaf Ca (13.52%), root P (17.75%), root K (12.80%), soil NH₄⁺-N (17.78%), available K (26.66%), Olsen-P (184.30%), easily extractable glomalin-related soil protein (39.26%), and soil organic carbon (16.25%) concentrations compared to the uninoculated treatment. Inoculation with <i>S. indica</i> also significantly up-regulated the expression of <i>CoHKT1;1</i> and <i>CoCAX1;2</i> in the leaves and roots and <i>CoPht1;1</i>, <i>CoPht1;2</i>, and <i>CoPht1;3</i> in the leaves. Plants inoculated with <i>S. indica</i> also presented significantly higher leaf glucose, fructose, and sucrose concentrations, accompanied by up-regulated expression of <i>CoSWEET2a</i>, <i>CoSWEET7</i>, <i>CoSWEET9b</i>, <i>CoSWEET17a</i>, and <i>CoSWEET17b</i>. These results suggest that <i>S. indica</i> has significant potential as a biostimulant for enhancing the growth and nutritional profile of <i>C. oleifera</i>, thereby contributing to sustainable oilseed production.