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Surface‐enhanced Raman spectroscopy (SERS) is known as a powerful technique to provide label‐free analyses for chemical sensing. While the sensitivity of SERS is heavily dependent on the prepared SERS functional substrates, finding appropriate substrates is critical to achieve sufficient signal enhancement. Meanwhile, the capture of targets to such effective enhancing region becomes challenging especially for low concentration. Here, we report a coupled Au/DNAs/Au gap plasmon platform that provides remarkable SERS enhancement and sensitivity for mercury ion sensing. Through nanoimprint lithography, large area of Au nanofingers can be fabricated with excellent uniformity and flexibility. These Au nanofingers can be further modified by DNA aptamers to construct Au/DNAs/Au gap plasmon nanostructures with well‐defined gap sizes. Due to the subnanometer gap size, strong interaction between collapsed Au nanofingers can be induced to create extremely enhanced near‐field with nanoscale mode volume. When Hg2+ ions exist around the platform, they can specially bind to thymine (T) bases of DNA and form T‐Hg2+‐T pairs between adjacent single strand DNAs. As a result, DNAs that initially lie on the Au surface are forced to stand in rigid duplex‐structures. This morphology change can be directly indicated by the ratio between adenine (A) and guanine (G) SERS signals to reveal the Hg2+ concentration. Given the strong field enhancement as well as the close distance between DNA and hotspot, ultralow Hg2+ concentration down to 10−9 M is successfully detected. This work demonstrates a SERS platform with high selectivity and sensitivity, which exhibits significant potential for molecule sensing applications. [ABSTRACT FROM AUTHOR]