The highly luminescent stable Eu3+ complex [Eu(tta)3(naptpy)] [1, tta = 2‐thenoyltrifluoroacetone, naptpy = 2‐{4‐[(2,2′:6′,2′′‐terpyridin)‐4′‐yl]phenyl}‐6‐bromo‐1H benzo[de]isoquinoline‐1,3(2H)‐dione] was designed as a pH‐responsive anion‐sensing probe. Complex 1 was synthesized and characterized by various physicochemical and spectral methods such as FTIR spectroscopy, 1H NMR spectroscopy, ESI‐MS, UV/Vis spectroscopy, and emission spectroscopy. A photosensitization mechanism occurs through two antennae (tta and naptpy) for Eu luminescence by energy transfer to the emissive 5D0 states of the Eu3+ ion through the corresponding excited triplet states of the ligands. Complex 1 displays strong luminescence, remarkable photostability, a long luminescence lifetime (τ = 0.365 ms) in H2O, and no coordinated inner‐sphere water molecules (q < 1). The pH‐dependent study showed that the emission intensity of 1 was quenched or “switched off” upon the displacement of the β‐diketonate antenna owing to the acid sensitivity of the α‐proton of tta and a subsequent ligand‐exchange reaction. The anion sensitivity of the probe was evaluated for a series of anions in aqueous N,N‐dimethylformamide (DMF). In the presence of citrate, F–, and HCO3– ions, the luminescence of 1 was quenched, whereas only minor luminescence changes were observed in the presence of other anions owing to the oxophilicity or stronger binding affinity of the hard oxoanions with the hard Lewis acidic Eu3+ ions. The probe shows efficient DNA (Kb = 6.53 × 104 m–1) and bovine serum albumin (BSA, KBSA = 5.09 × 105 m–1) binding affinities under physiological conditions. The quenching of the luminescence intensity of 1 upon interaction with calf thymus DNA (CT‐DNA) suggests that it could be used for luminescence‐based sensing applications.