We investigated the potential use of leptin as a wildlife management tool to monitor adiposity in American black bears (Ursus americanus). Body mass (BM), body condition index (BCI), serum leptin concentration, and percent of body fat were measured in semi-free ranging (SFR) adult male (South Dakota, Oct 2003–Jun 2004) and free-ranging (FR) adult females (Colorado and Wyoming, 1997–99) in fall, winter, spring, and summer. These variables were analyzed by simple and multiple linear regression models to determine their relative value as predictors of body fat proportion over a wide range of adiposity (7.5–48% of total BM). Log-transformed serum leptin concentration (ln leptin) was the most consistent single surrogate predictor of percent body fat when compared with BM and BCI. For both SFR and FR cohorts, ln leptin was strongly predictive of percent body fat within the range of adiposity between 7.5–35%. Moreover, the best predictors of body fat were multiple regression models that included ln leptin in both SFR males (AIC_c  =  75.97, ω  =  0.61) and FR females (AIC_c  =  85.07, ω  =  0.54). Therefore, serum leptin markedly improves the resolution and accuracy of common field estimates of body condition in black bears. Because it is strongly associated with and predictive of bioelectrical impedance assay (BIA) estimation of body fat, further investigation of serum leptin as a predictor of adiposity is warranted.