In water-cooled D–T fusion tokamaks such as the Joint European Torus (JET), neutron activation of flowing water produces short-lived radionuclides that generate a distributed high-energy gamma and neutron radiation source in the water cooling circuit. The energy and intensity of the source are crucial factors in the design of cooling and shielding systems in future water-cooled deuterium–tritium fusion tokamaks such as ITER. While several past experiments provide fusion-relevant experimental data on water activation, the JET water activation experiment carried out during the 2023 DD and DT campaigns is the first such experiment in a fusion tokamak environment. In the water activation experiment at JET, two types of scintillation detectors were used to measure gamma rays from activated water in the basement below JET Octant 4. The system successfully recorded and processed data from over 1500 JET pulses, including both DD and DT operations, providing a robust data set for analyzing N-16 activity in cooling water. This manuscript describes the preparation of the JET water activation experiment, the calibration of the detectors and the analysis of the experimental data.