This work presents a physical mechanism for the origin of the Cosmic Microwave Background (CMB) within the framework of Medium Space–Time Theory (SMT). Instead of interpreting the CMB as relic radiation from a hot primordial plasma, SMT describes it as equilibrium radiation generated by relaxation processes of the nonlinear Φ-medium in the early universe. The formation of the observed blackbody spectrum arises naturally from phase-coherent fluctuations and elastic energy redistribution in the medium. The model explains the isotropy, spectral shape, and acoustic features of the CMB without invoking cosmic inflation or dark matter. The results establish a direct dynamical connection between medium elasticity, defect relaxation, and large-scale cosmological radiation backgrounds.