We present a mathematically sharp and operationally grounded formulation of matter, mass, and inertia as emergent phenomena arising solely from the coupling of energy with spacetime. Building on the Spacetime Exclusion Principle (SEP) and spacetime porosity framework, we demonstrate that matter is not a fundamental substance but a stabilized configuration of energy constrained by the finite permeability, exclusion capacity, and elastic response of spacetime itself. Mass, inertia, and gravitational charge emerge as equivalent measures of spacetime–energy coupling strength. The framework preserves all experimentally validated predictions of General Relativity, Quantum Field Theory, and nuclear phenomenology while resolving conceptual tensions surrounding singularities, gravitons, confinement, and the ontological status of matter.