Maize is an important crop for the Portuguese agricultural sector. Future climate change, with warmer and dryer conditions in this Mediterranean environment, will challenge this high-water demanding crop. The present study aims at assessing the response of maize yield, growth cycle, seasonal water input and daily water productivity (DWP) to climate change, and analyse water-yield relations. For this purpose, two process-based crop models are used (STICS and AquaCrop) and were validated in simulating irrigated maize yields in Central Portugal (Ribatejo) by using regional statistics (1986–2005). Both models show an overall agreement in their outputs. The 2-model mean outputs are considered under future climate projections (2021–2080; RCP4.5 and 8.5), using the global/regional climate model chain M-MPI-ESM-LR/SMHI-RCA4. The most significant reductions on maize yield (−17%), growth cycle (−12%) and DWP (−19%) are observed for 2061–2080 under RCP8.5, with a noticeable decrease of seasonal water input (−9%) during 2041–2060. Decreased DWP is largely due to significant yield reduction, with limited benefit of atmospheric CO2 enrichment. A water-yield relation analysis highlights that an increase of 2–14% in irrigation for future scenarios (compared to 1986–2005) might be a suitable strategy to mitigate yield reduction, despite substantially lower DWP (down to −23%). These findings demonstrate that our model approach can be used as a decision support tool by Portuguese farmers, particularly in optimizing maize production under changing climates.