Mercury (Hg2+) is a highly toxic and widespread environmental pollutant. Herein, a regenerable and highly selective core-shell structured magnetic mesoporous silica nanocomposite with functionalization of thymine (T) and T-rich DNA (denoted as Fe3O4@nSiO(2)@mSiO(2)-T-TRDNA nanocomposite) has been developed for sinultaneous detection and removal of Hg2+. In this work, the thymine and T-rich DNA were immobilized onto the interior and exterior surface of outermost mesoporous silica, respectively. The detection mechanism is based on Hg2+-mediated hairpin structure formed by T-rich DNA functionalized on the exterior surface of the nanocomposites, where, upon addition of SYBR Green I 46, strong fluorescence is observed. In the absence of Hg2+, however, addition of the dye results in low fluorescence. The limit of detection for Hg2+ in a buffer is 2 nM by fluorescence spectroscopy. Simultaneously, the Fe3O4@nSiO(2)@mSiO(2)-T-TRDNA, nanocomposite features a selective binding with Hg2+ between two thymines immobilized at the interior surface of the mesopores and exhibits efficient and convenient Hg2+ removal by a magnet. Kinetic study reveals that the Hg2+ removal is a rapid process with over 80% of Hg2+ removed within approximately 1 h. The applicability of the developed nanocomposites is demonstrated to detect and remove Hg2+ from samples of Xiangjiang river water spiked with Hg2+. In addition, distinguishing aspects of the Fe3O4@nSiO(2)@mSiO(2)-T-TRDNA nanocomposites for Hg2+ detection and removal also include the regeneration using a simple acid treatment and resistance to nuclease digestion. Similar process can be used to functionalize the Fe3O4@nSiO(2)@mSiO(2) nanocomposites with other nucleic acids and small molecules for environmental and biomedical applications.