Abstract: To investigate the effects of water content on the energy evolution and the underlying mechanisms of rockburst in sandstone, red sandstone was selected as the test material. Specimens with different degrees of water saturation were prepared using a single-face water absorption method, and single-face unloading rockburst tests were conducted. Based on stress–strain responses, energy evolution characteristics, and acoustic emission (AE) behaviors, the initiation and instability processes of rockburst were systematically analyzed. The results indicate that: with increasing water content, the peak strength of sandstone gradually decreases while the peak strain increases, and the failure mode transitions from typical brittle behavior to relatively ductile behavior. The energy partitioning pattern shifts from elastic strain energy dominance to dissipation energy dominance, and the energy release mode evolves from abrupt and localized, release to gradual and distributed release, resulting in a significant reduction in rockburst intensity. AE characteristics reveal that unstable crack propagation is inhibited under higher water content, accompanied by a marked decrease in high-energy AE events. Mechanistically, water weakens inter-particle cementation, promotes energy dissipation through crack sliding, and alters crack propagation patterns, thereby regulating the energy storage capacity and transformation pathways of the rock mass. These effects collectively control the rockburst process. The findings provide new insights into the mechanisms governing rockburst in water-bearing rock masses and for improving engineering prevention and control strategies.