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We utilize synchrotron X-ray diffraction measurements, conducted inside a diamond anvil cell, to analyze the high-pressure stability of Mg/Nb multilayered nanocomposites of equal (1: 1) and unequal (1: 10) thickness ratios. At larger layer thicknesses, Mg in these nanocomposites exists in its traditional hexagonal close packed (hcp) structure, while below a critical layer thickness of 7–8 nm, the Mg structure is found to transform into an interface strain-induced metastable pseudomorphic body center cubic (bcc) crystal structure. The hcp Mg present in the larger layer thicknesses exhibits an hcp-to-bcc phase transformation at pressures greater than 44 GPa, and this pressure value is found to vary between the equal and unequal Mg/Nb nanocomposite thickness ratios. On the other hand, the pseudomorphic bcc Mg structure is stable up to pressures of 60 GPa. Additionally, the compressibility of the pseudomorphic bcc …