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Water stress differently affects yield and fruit quality in processing tomato, usually depressing crop productivity and improving the overall quality of fruits. These effects are well known but there is still a lack of information on water stress effects upon some endogenous enzymatic activities which can be considered as markers of fruit spoilage. Within them, pectinases and oxidases are the major enzymes involved in fruit quality decline. The pectinases are responsible for fruit firmness losses (softening), due to cell walls autolysis and pectin degradation; polyphenol oxidase affects phenols inducing an undesired fruit coloration (browning) followed by losses in nutritional value of tomato fruits. This research was carried out in Sicily (South Italy), under a typical semiarid climate, aiming at assessing pectin methylesterase (PME, EC 3.1.1.11), polygalacturonase (PG, EC 3.2.1.15) and polyphenol oxidase (PPO, EC 1.14.18.1) activity in processing tomato cv. Brigade, cultivated under soil water deficit, as compared to a fully irrigated crop (100% ETc restoration). Water stress in unirrigated tomato decreased PME, PG and PPO activity by 25, 20 and 49% respectively, as compared to the well irrigated tomato, with positive implications in fruit quality maintenance. Water stress conditions allowed to obtain fruits with increased total solids (+23%), total soluble solids (+5%), firmness (+27%) and antioxidant activity (+12%). The results highlight that, although irrigation is required in processing tomato to obtain adequate yields, water shortage improves fruit quality and allows irrigation water saving which is an important goal for semi-arid areas.