Along the years, ruthenium oxides have been deeply investigated for their magnetic or catalytic properties. The exploration of new synthesis processes, and especially low-temperature ones, is of primary importance to obtain new materials with interesting properties. Here, we highlight the tunability of a low-temperature (200 °C) hydrothermal synthesis route of alkaline-earth ruthenates. Changing only simple physicochemical parameters such as pH, it is possible to obtain a large diversity of metastable compounds. Among them, four compounds, namely, BaRu2O6, Ba2Ru3O10, Ba5Ru4O15, and Sr2Ru3O9(OH) are reported here for the first time. The influence of the reaction parameters (temperature, counter ions, reactant ratio, and pH) is studied. According to these observations, the importance of the reaction pH is highlighted and a reaction scenario is proposed. Finally, the crystal structures of Ba2Ru3O10, Ba5Ru4O15, and Sr2Ru3O9(OH) are reported. These findings further highlight the richness of low-temperature chemistry in the discovery of new metastable phases to be further explored by scientists.