A 2-year study was conducted to examine the impact of deficit irrigation on dry biomass, water-use efficiency (WUE), fruit yield and quality in open-field processing tomato at high plant density in a semi-arid environment. Three irrigation treatments (nil; and 100% (full) and 50% (deficit) restoration of crop evapotranspiration (ETc), respectively) and two plant densities (2.5 (P1) and 5.0 (P2) plants m^–2) were studied. Dry biomass and fruit yield per plant were lower in P2 than in P1, but at high plant density the crop compensated for biomass and yield decrease at the plant level. Fruit yield in P2 was greater than that in P1, by 36% in 2004 and 33% in 2005. Water limitation improved quality traits compared with full irrigation. Deficit irrigation, especially in P2, enhanced WUE and allowed a water saving of >45% relative to full irrigation, while keeping high levels of fruit quality. The yield response factor, Ky, which correlates relative fruit yield losses to relative ETc reduction, was higher (0.63) than Kss (0.44), which correlates relative total dry biomass losses to relative ETc reduction, revealing a greater crop sensitivity to soil-water deficit in terms of fruit yield than dry biomass. Therefore, Ky may of use in identifying the plant density at which water productivity is maximised or yield losses are minimised.