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 µmol CO₂ m⁻² s⁻¹), while stomatal conductance (0.14–0.15 mol H₂O m⁻² s⁻¹) and WUE (4.0 µmol CO₂ mmol⁻¹ H₂O) were not affected by SAP. Total soluble solids increased under severe deficit (7.8 °Brix at 25% WC) and were enhanced by SAP (from 6.9 to 7.6 °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.