Objective Brown adipose tissue (BAT) generates heat in response to cold, and low BAT activity has been linked to obesity. However, recent studies are inconclusive as to whether BAT is involved in diet-induced thermogenesis and mitigates weight gain from prolonged overeating. We therefore investigated whether BAT activity is related to metabolic adaptation arising from 8 weeks of overfeeding in humans. Methods Fourteen men (aged 24±3 years, BMI 24.5±1.6 kg/m2) were overfed by 40% for 8 weeks. Before and after, energy expenditure and metabolic adaptation were measured by whole-room respiratory calorimetry. A marker of BAT activity was measured using infrared imaging of the supraclavicular BAT depot. Results At the end of 8 weeks of overfeeding, metabolic adaptation—defined as the percent increase in sleeping energy expenditure beyond that expected from weight gain—rose from −0.9 ± 3.9% to 4.7±5.6% (p=0.001). However, BAT thermal activity was unchanged (p=0.81). Moreover, BAT thermal activity did not correlate with the degree of metabolic adaptation (p=0.32), nor with the change in body weight (p=0.51). Conclusions BAT thermal activity does not change in response to overfeeding, nor does it correlate with adaptive thermogenesis. Our data suggest that BAT does not mediate metabolic adaptation to overeating in humans.