We report the development of a new diagnostic capable of measuring plasma ion distributions as a function of all three velocity–space coordinates. The diagnostic makes use of laser‐induced fluorescence (LIF) and computer‐assisted image reconstruction techniques. LIF yields high‐resolution, nonperturbing measurements of one‐dimensional distributions that are integrated in two directions through three‐dimensional velocity space. Computer tomography allows for the unambiguous determination of the complete ion velocity distribution. In addition to a description of the diagnostic, examples of recovered distributions obtained from experiments are given, and the effects of the major steps in the data processing are discussed.