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In deep underground geotechnical engineering, water–rock interaction emerges as a critical factor influencing the stability of engineering rock masses. This study delves into the macroscopic nonlinear mechanical behavior and fracture characteristics of natural water-bearing and saturated sandstone samples under various true triaxial stress states. The objective is to systematically elucidate the impact of actual three-dimensional stress conditions and water content on the mechanical behavior and fracture mechanisms of sandstone formations. The findings reveal significant anisotropic deformation in sandstone under 3D stress, while saturated sandstone exhibits robust plastic deformation capacity. Moreover, changes in the strain ratio coefficient are identified as precursors to rock fracture. The dilatation behavior of sandstone weakens progressively with increasing σ2, indicating reduced expansion in saturated sandstone. To account for the influence of σ2, a novel dilation index is proposed. The true triaxial strength of sandstone demonstrates a close relationship with both σ2 and water content, with strength initially increasing before declining as σ2 rises. In addition, water significantly undermines sandstone strength, although this weakening trend moderates with increasing σ2. Morphological analysis and SEM observations unveil three-dimensional volume fracturing characteristics in rock fractures, primarily characterized by shear accompanied by tensile fractures. Saturated sandstone samples, in contrast, exhibit numerous tensile and intergranular fractures. Overall, this study holds paramount practical significance for the evaluation of rock stability and the design of water-bearing engineering rock masses, encompassing domains, such as geology, water conservancy, and mining. [ABSTRACT FROM AUTHOR]