We synthesised aza‐12‐crown‐4 ether‐substituted naphthalene diimide (NDI‐12‐C‐4) and demonstrated its properties as an fluorescent and colorimetric sensor. The NDI‐12‐C‐4 uses the aza‐12‐crown‐4 moiety to bind Li+ ion selectively over larger monovalent cation such as Cs+, Na+ and K+. The sensor is chemically stable due to the crown‐ether, and the appended NDI fluorophore allows leads to optical changes exclusively for the monovalent Li+ ion. UV–vis absorption, fluorescence, cyclic voltammetry (CV), 1H NMR and naked eye‐detection clearly demonstrate selective, sensitive binding of Li+ ion with a detection limit of 5.0 μM. Importantly, reversible binding of Li+ by NDI‐12‐C‐4 was also observed upon addition of EDTA, after which the sensor could be recovered via simple centrifugation, indicating suitability for use in a biological sensor device.