In many plants, glycine betaine acts as an osmotic protectant and osmolyte. But some plants, such as the tomato, are not able to synthesize and accumulate this osmolyte. This study was designed to investigate the effect of exogenous application of glycine betaine in reducing drought stress in tomato plants. Water stress in three levels (0, 3 days and 5 days without irrigation) and glycine betaine in three levels (0, 25 and 50 mM as foliar spraying) was applied. water stress in the both levels reduced the third leaf area, shoot dry weight, percent water content of the third leaf, the amount of chlorophyll, carotenoids, proteins, phenolic compounds and anthocyanins and increased lipid peroxidation, proline, reducing sugars and total ascorbate. However, application of glycine betaine in two levels, decreased lipid peroxidation, proline content and increased the amount of chlorophyll, carotenoids, reducing sugars, proteins, phenolic compounds, anthocyanins and total ascorbate and thus, increased the third leaf area, shoot dry weight and percentage of third leaf water. The results of this study showed that, exogenous application of two levels of glycine betaine in tomato plants, unable to synthesize this substance, is effective in reducing the effects of drought stress and improve growth parameters under stress and non-stress conditions.