Naphthalene‐ and coumarin‐substituted dithiolene ligands were synthesized, and their coordination behavior towards different metal ions, including biologically relevant ions like Cu+, Cu2+, and Fe3+, was tested. Both ligands bind nickel selectively and show no evidence of binding any other metal investigated. Particularly, the compound bearing the fluorophore coumarin can serve as a colorimetric (“naked eye”) and on‐off fluorometric marker for the detection of Ni2+ ions with a characteristic near‐infrared (NIR) signature even in the presence of various transition‐metal ions, which are normally difficult to differentiate from nickel. Competitive fluorescence experiments showed that the concomitant presence of Ag+, Al3+, Ca2+, Cd2+, Co2+, Cu+, Cu2+, Fe3+, Hg2+, Mg2+, Mn2+, Pb2+, Sn2+, Zn2+, and MoO42– could not inhibit or diminish the Ni2+‐specific fluorescence quenching, emphasizing the inherent Ni2+ ion selectivity of the system. The coordination behavior of the marker and Ni2+ were studied in detail by fluorescence titration experiments in THF/MeOH solution. X‐ray structures were determined to complement characterization of the precursors and the naphthalene–dithiolene–nickel complex.