Fluorescent biosensors that facilitate reagentless sensitive detection of small molecules are crucial tools in the areas of therapeutics and diagnostics. However, construction of fluorescent biosensors with desired characteristics, that is, detection wavelengths and concentration ranges for ligand detection, from macromolecular receptors is not a straightforward task. An ATP-binding ribonucleopeptide (RNP) receptor was converted to a fluorescent ATP sensor without chemically modifying the nucleotide in the ATP-binding RNA. The RNA subunit of the ATP-binding RNP and a peptide modified with a pyrenyl group formed a stable fluorescent RNP complex that showed an increase in the fluorescence intensity upon binding to ATP. The strategy to convert the ATP-binding RNP receptor to a fluorescent ATP sensor was applied to generate fluorescent ATP-binding RNP libraries by using a pool of RNA subunits obtained from the in vitro selection of ATP-binding RNPs and a series of fluorophore-modified peptide subunits. Simple screening of the fluorescent RNP library based on the fluorescence emission intensity changes in the absence and presence of the ligand afforded fluorescent ATP or GTP sensors with emission wavelengths varying from 390 to 670 nm. Screening of the fluorescence emission intensity changes in the presence of increasing concentrations of ATP allowed titration analysis of the fluorescent RNP library, which provided ATP sensors responding at wide concentration ranges of ATP. The combinatorial strategy using the modular RNP receptor reported here enables tailoring of a fluorescent sensor for a specific ligand without knowledge of detailed structural information for the macromolecular receptor.