Developments in communication technologies depend on the parallel progress in materials innovation, data processing devices, and their strategic integration. Terahertz (THz) science and technology is the latest field to witness phenomenal growth in 6G communication and quantum materials devices. Such advancements depend on the ability to control both the amplitude and phase-shift of THz radiation, with the latter being particularly crucial. Currently, free-space THz phase shifters exploit the intrinsic changes in carrier density, resulting in a weak response that can be amplified by metamaterial structures, but at the expense of a significantly reduced bandwidth. In this work, we demonstrate a novel mechanism that leverages only the intrinsic property of magnetoelastic coupling to induce a giant THz phase modulation. An unprecedentedly large phase-shift of 3.35π radians at ∼0.9 THz occurs during the magnetoelastic phase transition between 72 and 35 K in Ba3BiIr2O9. This is accompanied by a remarkable figure-of-merit that is four to five times greater and spans a significantly broader spectral range than that of other above π/2 free-space modulators. Corroborated by theoretical calculations, we show that the spin–phonon coupling dynamics have a defining influence in altering the dielectric function that underlies these properties. These findings present the prospect of integrating magnetoelastic quantum materials in emergent THz communication tools, which rely on phase-shift modulation for information processing.