Nuclear fusion requires overcoming or traversing a repulsive Coulomb barrier of hundreds of kiloelectronvolts, rendering the probability of fusion at sub-keV energies vanishingly small. Yet in condensed matter, the electronic and structural environment of reacting nuclei can profoundly alter fusion rates. Here we demonstrate that deuterium-deuterium fusion within metallic foils exhibits a pronounced reaction yield plateau (i.e., a finite, non-vanishing yield floor) below 2 keV-in stark contrast to the expected exponential suppression at low energy. At the lowest energies measured, this corresponds to fusion yields enhanced by more than 10¹⁸ relative to bare-nucleus (unscreened) expectations. Using a dual-chamber platform that combines electrochemical deuterium loading with low-energy ion-beam bombardment, we observe this behavior in both palladium and titanium hydrides. These results reveal a previously unrecognized regime of low-energy nuclear reactions in solids, demonstrating that materials degrees of freedom can fundamentally renormalize tunneling probabilities and fusion cross-sections.