Recent advances in adsorbents have improved the removal of mercury ions from wastewater. Metal-organic frameworks (MOFs) have been increasingly used as adsorbents due to their high adsorption capacity and ability to adsorb various heavy metal ions. UiO-66 (Zr) MOFs are mainly used because they are highly stable in aqueous solutions. However, most functionalized UiO-66 materials are unable to achieve a high adsorption capacity because of the undesired reactions that occur during post-functionalization. Herein, we report a facile post-functionalization method to synthesize a MOF adsorbent with fully active amide- and thiol-functionalized chelating groups, termed UiO-66-A.T. UiO-66-A.T. was synthesized via a two-step reaction by crosslinking with a monomer containing a disulfide moiety, followed by disulfide cleavage to activate the thiol groups. UiO-66-A.T. removed Hg2+ from water with a maximum adsorption capacity of 691 mg g-1 and a rate constant of 0.28 g mg-1 min-1 at pH 1. In a mixed solution containing 10 different heavy metal ions, UiO-66-A.T. has a Hg2+ selectivity of 99.4%, which is the highest reported to date. These results demonstrate the effectiveness of our design strategy for synthesizing purely defined MOFs to achieve the best Hg2+ removal performance to date among post-functionalized UiO-66-type MOF adsorbents.