The influence of endophyte fungus piriformospora indica on characteristics of the growth, water status, photosynthetic pigments concentration, gas exchange, and chlorophyll fluorescence of tomato plants under salt stress (0, 50, 100 and 150 mM) was studied in the greenhouse. Under salt stress, mycorrhizal tomato plants had higher dry weight of shoot and root, higher height, higher carotenoid and chlorophyll content, better water status (increased water use efficiency, relative water content and water potential), higher gas exchange capacity (increased photosynthetic rate, stomatal conductance and transpiration rate, and decreased intercellular CO2 concentration), higher non-photochemistry efficiency [increased non-photochemical quenching values (NPQ)], and higher photochemistry efficiency [increased the maximum quantum yield in the dark-adapted state (Fv/Fm), the maximum quantum yield in the light-adapted sate (Fv′/Fm′), the effective quantum yield of photochemical energy conversion of PSII (∆F'/F'm) and the photochemical quenching values (qP)], compared with non-mycorrhizal tomato plants. All the results indicated that endophyte symbiosis alleviates the deleterious effect of salt stress probably via improving photosynthesis and water status of tomato plants.