<p>Water scarcity and inefficient irrigation are major constraints on crop production. Regulated deficit irrigation (RDI) offers a promising solution; however, the lack of practical procedures limits its widespread adoption. This study developed an RDI strategy for greenhouse-grown melons (<i>Cucumis melo</i> L.), aiming to optimize the timing and amount of irrigation. To quantify melon water demand by using the crop coefficient–reference evapotranspiration approach, crop coefficients were estimated for each growth stage under representative cultivation conditions in Taiwan. In parallel, growth-stage-specific stomatal conductance thresholds were identified, and corresponding models for classifying water status were developed. In the proposed strategy, the water status classification models are used to determine irrigation timing; irrigation is performed with 100% and 35%–60% of the estimated crop evapotranspiration before and after fruit setting, respectively. The proposed strategy was validated across three crop seasons in soilless and soil-grown systems. In both systems, it resulted in substantially higher water productivity than conventional practices did. In soil-grown systems, the total irrigation amounts were reduced by 19.3% to 25.7% without yield or fruit quality being compromised. In soilless systems, the strategy increased fruit sweetness and reduced fruit cracking, although it also led to lower yield and smaller fruits. Multicriteria decision analysis through the Technique for Order Preference by Similarity to Ideal Solution confirmed that the proposed strategy mostly achieved a more favorable balance among yield maintenance, fruit quality enhancement, and water saving than did conventional irrigation. This strategy has the potential to enhance water management in greenhouse melon cultivation, particularly in soil-grown systems.</p>