Amorphous MoO3−x nanosheets were fabricated by the room‐temperature oxidation of molybdenum powder with H2O2, followed by light‐irradiation reduction in methanol. When applied as a substrate for surface‐enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO3−x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H2O2‐based Fenton‐like reaction. More importantly, the non‐invasive SERS was unprecedentedly used for the self‐diagnosis of amorphous MoO3−x as a more selective photocatalyst than its crystalline counterpart.