An aqueous room-temperature phosphorescent (RTP) probe for Gd3+ is reported based on Gd3+-induced intersystem promoting effect and the oxygen-shielding property of the Gd3+/AMP/fluorescein coordination polymer nanoparticles (CPNs). Besides selective Gd3+ detection, the results are important for harnessing the advantages of RTP detection in an aerated aqueous solution, which is difficult with current phosphors.