The Mediterranean region (MR) includes the largest semi-enclosed sea on Earth and is an area of both exceptional biodiversity value and intense and increasing human activities. MR has a unique character as it is in a transition zone between temperate, cold mild-latitudes and the tropics with several large-scale atmospheric oscillations/teleconnection patterns. This determines a high temporal variability of climate which causes periods of excess water with widespread floods followed by long drought episodes and heat waves, making the region highly vulnerable to hydrological extremes. Therefore, resolving the water cycle over the MR is central for protecting people and guaranteeing water and food security. Previous efforts to resolve the water cycle in the MR have mainly used model outputs or reanalysis and in situ data networks. In this context, the European Space Agency (ESA) has supported significant scientific efforts to advance the way we can observe and characterise the Mediterranean water cycle from satellites with Watchful, Irrigation+, and WACMOS-Med projects. For instance, the WACMOS-Med considered several novel techniques to estimate the different components of the Mediterranean water cycle estimated by satellite observations while minimising the residual errors. WACMOS-Med provided a rational assessment of the different limitations of current satellite technology to characterise in a consistent and accurate manner the different components of the water cycle. However, limitations associated to resolution in space and time, accuracies, uncertainty definition and inter-product consistency hinder the practical use of the products for operational application in several domains (e.g., agriculture, water resource management, hydro-climatic extremes and geo-hazards) over the MR.  Here we present a new ESA project “4DMED-Hydrology” which aims at developing an advanced, high-resolution, and consistent reconstruction of the Mediterranean terrestrial water cycle by using the latest developments of Earth Observation (EO) data as those derived from the ESA-Copernicus missions. In particular, by exploiting previous ESA initiatives, 4DMED-Hydrology intends 1) to show how this EO capacity can help to describe the interactions between complex hydrological processes and anthropogenic pressure (often difficult to model) in synergy with model-based approaches; 2) to exploit synergies among EO data to maximize the retrieval of information of the different water cycle components (i.e., precipitation, soil moisture, evaporation, runoff, river discharge) to provide an accurate representation of our environment and advanced fit-for-purpose decision support systems in a changing climate for a more resilient society.  We organize the project in four consequent steps: 1) developing high-resolution (1 km, daily, 2015–2021) EO-based datasets of the different components of the water cycle by capitalizing on Sentinel missions’ capabilities and previous ESA projects; 2) merging these datasets to obtain land water budget closure and providing a consistent high-quality merged dataset; 3) addressing major knowledge gaps in water cycle sciences enhancing our fundamental scientific understanding of the complex processes governing the role of the MR in the Earth and climate system with the water cycle; 4) transferring novel science results into solutions for society via four user-oriented case studies focusing on flood and landslide hazard, drought and water resources management by involving operational agencies, public institutions and economic operators in the MR.​ 4DMED-Hydrology will focus on four test areas, namely the Po river basin in Italy, the Ebro River basin in Spain, the Hérault River basin in France and the Medjerda River basin in Tunisia, which are representatives of climates, topographic complexity, land use, human activities, and hydrometeorological hazards of the MR. The developed products will be then extended to the entire region. The resulting EO-based products (i.e., experimental datasets, EO products) will be distributed in an Open Science catalogue hosted and operated by ESA.