Obesity and its metabolic consequences are a major cause of morbidity and mortality. Brown adipose tissue (BAT) utilizes glucose and free fatty acids to produce heat, thereby increasing energy expenditure. Effective evaluation of human BAT stimulators is constrained by the current standard method of assessing BAT—PET/CT—as it requires exposure to high doses of ionizing radiation. Infrared thermography (IRT) is a potential noninvasive, safe alternative, although direct corroboration with PET/CT has not been established. Methods: IRT and 18F-FDG PET/CT data from 8 healthy men subjected to water-jacket cooling were directly compared. Thermal images were geometrically transformed to overlay PET/CT-derived maximum intensity projection (MIP) images from each subject, and the areas with the most intense temperature and glucose uptake within the supraclavicular regions were compared. Relationships between supraclavicular temperatures (TSCR) from IRT and the metabolic rate of glucose uptake (MR(gluc)) from PET/CT were determined. Results: Glucose uptake on MR(gluc)MIP was found to correlate positively with a change in TSCR relative to a reference region (r2 = 0.721; P = 0.008). Spatial overlap between areas of maximal MR(gluc)MIP and maximal TSCR was 29.5% ± 5.1%. Prolonged cooling, for 60 min, was associated with a further TSCR rise, compared with cooling for 10 min. Conclusion: The supraclavicular hotspot identified on IRT closely corresponded to the area of maximal uptake on PET/CT-derived MR(gluc)MIP images. Greater increases in relative TSCR were associated with raised glucose uptake. IRT should now be considered a suitable method for measuring BAT activation, especially in populations for whom PET/CT is not feasible, practical, or repeatable.