Developing Ag-based surface-enhanced Raman spectroscopy (SERS) sensors for detecting Hg(II) has garnered significant research interest due to their unparalleled selectivity, which is brought by the specific Ag-Hg amalgamation reaction. However, existing sensors perform unsatisfactorily in the trace detection of Hg(II) because the low concentration of Hg(II) does not have the redox potential sufficient to amalgamate with Ag. To address this challenge, a plasmonic MOF SERS sensor is developed, nanoetched Ag@UiO-68-SMe, by integrating the enormous Raman enhancement effects of nanoetched Ag with the selective enrichment function of UiO-68-SMe into single entity. This sensor enables on-site readout of Hg(II) in various real-world samples with high selectivity and sensitivity (0.17 ppb) using a portable Raman spectrometer coupled with a homemade 3D print holder. Mechanistic studies reveal that the UiO-68-SMe selectively captures and concentrates trace amounts of Hg(II) through thiomethyl groups, significantly increasing their redox potential. The resultant higher oxidative capacity allows for the spontaneous Ag-Hg amalgamation, inducing a SERS turn-off response to Hg(II), which is otherwise thermodynamically prohibited. This work not only reports a powerful SERS sensor for monitoring trace levels of Hg(II) pollution but also offers a proof-of-concept demonstration of utilizing the enrichment capabilities of MOF to manipulate redox reaction.