With the increasing applications of nucleic acid aptamers as a new class of molecular recognition probes in bioanalysis and biosensor development, the development of general and simple signaling strategies to transduce aptamer-target binding events to detectable signals is demanding. We have developed a new signaling method based on aptamers and a DNA molecular light switching complex, [Ru(phen)2(dppz)]2+, for sensitive protein detection. In this work, we have demonstrated the applicability of this signaling mechanism to small-molecule detection using ATP as a model target. Our results have shown that upon ATP binding to the folded aptamer where [Ru(phen)2(dppz)]2+ intercalated, the conformational change or distortion of the aptamer is large enough to cause a significant luminescence change of [Ru(phen)2(dppz)]2+. By monitoring the ATP-dependent luminescence intensity change, we have achieved ATP detection with high selectivity and high sensitivity down to 1 nM in homogeneous solution. The method is very simple without the needs for covalently labeling aptamers or using costly enzymes and multistep analysis as other reported fluorescence/luminescence assays for ATP. The successful detection of ATP indicates that using the signaling aptamers with [Ru(phen)2(dppz)]2+ is expected to be a general method for aptamer-based target detection.