Seasonal variation in resting energy expenditure (REE) is still under debate. This study investigated seasonal changes in REE and relevant factors among Korean adults. A total of 867 healthy volunteers (385 men and 482 women) aged 20-69 years were divided into four seasonal groups and subgroups based on age, body mass index (BMI), and percent body fat (PBF) quartiles. REE, body composition, glucose metabolism, thyroid hormones, and catecholamines were measured. The seasonal factor contributed to REE independent of anthropometric indices, with additional variation decreasing from 6% to 2% among younger and older persons, respectively. The adjusted REE in the winter was 5.4-13.9%, 7.8-14.3%, and 8.6-11.9% higher than that in the summer in the age, BMI, and PBF subgroups, respectively. T3 and log-transformed norepinephrine (NElog) were higher, whereas log-transformed epinephrine (EPIlog) was lower in the winter compared to the summer. The magnitude of the winter-summer difference in REE and T3 and of the summer-winter difference in EPIlog were reduced three-fold between the lowest and highest intervals of age and PBF, whereas the difference in NElog was constant across all age and PBF intervals. There was no obvious change in seasonal differences in REE or its relevant biomarkers across BMI intervals. In summary, season is an independent predictor of REE and its effect is attenuated by the increment of age and PBF but not BMI.