The efficient use of phosphate fertilizers and optimization of the amounts of irrigation water can maximize tomato growth and fruit production. This study aimed to evaluate the effects of different phosphorus (P) doses and sources on the growth and production components of tomato plants of the cultivar Gaúcho Melhorado Nova Seleção subjected to different irrigation water percentages. To achieve this, we set up an experiment using a factorial design to test the effects of four doses of P₂O₅ (corresponding to 25%, 50%, 100%, and 200% of the recommended dose), two P sources (monoammonium phosphate—MAP and organomineral—OM), and four irrigation water percentages (50%, 75%, 100%, and 125% of field capacity). Tomato plant growth improved when water was supplied at a percentage close to 100% of field capacity, with increased plant height, leaf length, and number of flowers observed (increases of 11.95%, 7.33%, and 13.87%, respectively, compared to 50% of field capacity). However, both excess and deficit irrigation resulted in morphological changes in tomato plants. Additionally, we observed that OM was more effective than MAP in increasing plant diameter and number of flowers, with increases of up to 36.4% and 227.6%, respectively, when using OM. Conversely, tomato growth was negatively affected by higher doses of MAP doses, suggesting that 25% of the recommended dose may yield the best growth rates. We verified that tomato plants can compensate for low phosphorus doses by increasing productivity with higher water amounts (125%–42.40 t ha⁻¹), but high phosphorus doses result in greater fruit production with lower water percentages (50%–41.52 t ha⁻¹).