Measuring the levels of Fe3+ in human body has attracted considerable attention for health monitoring as it plays an essential role in many physiological processes. In this work, we reported a selective fluorescent nanoprobe for Fe3+ detection in biological samples based on ultrabright N/P codoped carbon dots. By employing adenosine 5'-triphosphate (ATP) as the carbon, nitrogen, and phosphorus source, the N/P codoped carbon dots could be simply prepared through hydrothermal treatment. The obtained carbon dots exhibited high quantum yields up to 43.2%, as well as excellent photostability, low toxicity, and water solubility. Because of the Fe-O-P bonds formed between Fe3+ and the N/P codoped carbon dots, this nanoprobe showed high selectivity toward Fe3+ against various potential interfering substances in the presence of EDTA. The fluorescence quenching of as-fabricated carbon dots was observed with the increasing Fe3+ concentration, and the calibration curve displayed a wide linear region over the range of 1-150 μM with a detection limit of 0.33 μM. The satisfactory accuracy was further confirmed with the river samples and ferrous sulfate tablets, respectively. With the above outstanding properties, these N/P codoped carbon dots were successfully applied for direct detection of Fe3+ in biological samples including human blood serum and living cells. As compared to the most reported carbon dots-based Fe3+ sensors, this nanoprobe showed high fluorescence, good accuracy, and excellent selectivity, which presents the potential practical application for diagnosis of Fe3+ related disease.