Water scarcity is increasingly challenging greenhouse tomato production, particularly in Mediterranean and semi-arid regions where irrigation water availability is becoming progressively limited. This study evaluated whether a superabsorbent polymer (SAP) can support water-saving irrigation in tomato grown in coconut fibre. Plants were cultivated in pots under four irrigation amounts (100, 75, 50, and 25% of crop water requirement-WC) combined with two SAP levels (0 and 2 g L-1). Irrigation was managed by a lysimetric control system. Reducing irrigation decreased total fruit yield (averaged across SAP treatments) from 100% WC (1212 g plant-1) to 50-25% WC (914 and 624 g plant-1, respectively), while non-marketable fruit number was unchanged (15.4 fruit plant-1, on average). SAP increased total yield, averaged across irrigation treatments (from 925 to 1022 g plant-1), and marketable fruit number (from 26.3 to 32.3 fruit plant-1), without affecting unitary fruit weight (20.4 g fruit-1, on average). SAP also increased net photosynthesis (from 16.0 to 17.4 & micro;mol CO2 m-2 s-1), while stomatal conductance (0.14-0.15 mol H2O m-2 s-1) and WUE (4.0 & micro;mol CO2 mmol-1 H2O) were not affected by SAP. Total soluble solids increased under severe deficit (7.8 degrees Brix at 25% WC) and were enhanced by SAP (from 6.9 to 7.6 degrees Brix), while colour parameters were mainly driven by irrigation. Overall, the irrigation amount was the primary driver of performance. Moderate deficit irrigation (75% WC) maintained a marketable fruit number and total fruit weight comparable to full irrigation (100% WC). SAP amendment acted as a complementary tool to improve marketable production and net photosynthesis across irrigation levels, providing an additive benefit to crop productivity.