An important step on the way to future fusion power plants was the 2021 deuterium–tritium experimental campaign (DTE2) at the Joint European Torus (JET), in which crucial DT physics was investigated. In this study, we have reconstructed the fast-ion deuterium distribution function in JET discharge 99971 which broke the former fusion energy record. It is the first time that the fast-ion distribution has been reconstructed from experimental data in a DT discharge. The reconstruction shows that the fast-ion deuterium distribution is anisotropic, with a bias towards co-going ions (p > 0). The fast-ion deuterium distribution likely peaks in energy (E) at around –70 keV and has a marginal high-energy tail ( keV). Furthermore, an orbit analysis shows that the fast-ion distribution is composed of mostly co-passing orbits (), trapped orbits () and counter-passing orbits (), as well as a small population of potato orbits () and counter-stagnation orbits (). The orbit-type constituents of the neutron measurements are distributed in similar fractions.