Steel-Reinforced Resin (SRR) is a particulate material originally developed as an injectant for anchoring applications. More recently, it has been proposed as a filler material for cavities embedding mechanical connectors in FRP–steel hybrid bridges. In this context, the compressive behaviour of SRR becomes critical due to the multiaxial stress states and fatigue demands at a joint scale. This paper presents a comprehensive experimental and numerical investigation of SRR under monotonic, incremental cyclic, and fatigue compressive loading in unconfined conditions. A custom triaxial setup is also used to evaluate pressure sensitivity and strength enhancement due to confinement under monotonic loading. In parallel, micromechanical finite element models are developed to simulate the interactions between the resin matrix and the steel balls at the microscale, incorporating interface damage, friction, and cohesive failure. The models reproduce the observed nonlinear behaviour and reveal distinct Poisson's ratio responses in tension and compression, offering deeper insight into the mechanisms governing stiffness degradation, strain softening, and plateau behaviour.