In this paper, a sensitive, easy, efficient, and suitable method for the calculation of K f values of complexation between one derivative of Dansyl chloride [5-(dimethylamino) naphthalene-1-sulfonyl 4-phenylsemicarbazide] (DMNP) and Lanthanide(III) (Ln) ions is proposed, using both spectrofluorometric and spectrophotometric methods. Determination of K f showed that DMNP was mostly selective towards the erbium (III) ion. The validity of the method was also confirmed calculating the Stern–Volmer fluorescence quenching constants (K sv ) that resulted in the same consequence, obtained by calculating the K f of complexation values. In addition, the UV–vis spectroscopy was applied for the determination of K f only for the Ln ions that had interactions with DMNP. Finally, the DFT studies were done on Er 3+ and the DMNP complex for distinguishing the active sites and estimating the pair wise interaction energy. It can be concluded that this derivative of Dansyl chloride with inherent high fluorescence intensity is a suitable reagent for the selective determination of the Er 3+ ion which can be used in constructing selective Er 3+ sensors.