In a field experiment, processing tomato plants inoculated with arbuscular mycorrhizae (AM) and non-inoculated (Control) were supplied with three levels of watering. The AM inoculation significantly increased tomato root colonization regardless of the water supply levels. Under water deficit conditions, AM inoculation significantly increased the biomass production (from 1,189 to 2,062 g plant^-1). AM inoculation increased the phosphorus uptake in water deficit supply (from 0.5 to 1.3 g plant^-1) and in optimum water supply (from 0.3 to 0.6 g plant^-1). Photosynthesis was not affected by irrigation, but mycorrhizal inoculation enhanced the efficiency of photosystem II at all water levels. Inoculated plants accumulated less proline, potassium, and magnesium in shoots in response to water stress. Less organic and inorganic solutes in shoots of inoculated plants were accompanied by higher water use efficiency, better stomatal conductance, and higher leaf water potential. In conclusion, AM inoculation enabled host plants to alleviate moderate water stress, modulating the physiological status of the plants for better water exploitation.