Metal recognition by nucleic acids provides an intriguing route for biosensing of metal. Toward this goal, a key prerequisite is the acquisition of nucleic acids that can selectively respond to specific metals. Herein, we report for the first time the discovery of two small DNAs that can specifically bind Ni2+ and discriminate against similar ions, particularly, Co2+. Their minimal effective constructs are 60-70 nucleotides (nt) in length with Ni2+ binding even at harsh denaturing conditions of 8 M urea and 50 mM EDTA. Using isothermal titration calorimetry (ITC), we estimated the dissociation constant (KD) of a representative DNA to be 24.0 ± 4.5 μM, with a 9:1 stoichiometry of Ni2+ bound to DNA. As being engineered into nanosized particles, these DNAs can act like nanosponges to specifically adsorb Ni2+ from artificial wastewater, demonstrating their potential as a novel molecular tool for high-quality nickel enrichment and detection.