Objective: Accelerometer-determined physical activity, sedentary behavior, and energy expenditure estimation may be influenced by the accuracy of accelerometer-assessed inclinometer. The purpose of this study is to examine the accuracy of the ActiGraph GT9X (most recent ActiGraph model) inclinometer across participant body habitus, and in particular, differences in central adiposity. Methods: Eighteen participants (Mage = 21.5 years) completed a series of 11 consecutive activities that varied in both movement (i.e., no movement, walking, and pedaling) and anatomic position (i.e., lying, sitting, and standing). During the trials, the participants wore an ActiGraph GT9X accelerometer and direct observation is used to compare body position determination derived from the ActiGraph GT9X inclinometer. Results: The ActiGraph GT9X inclinometer demonstrated moderate accuracy (>68%) in body position determination for sedentary behaviors, high accuracy (>95%) in body position determination for ambulatory-based activities (e.g., walking and jogging), and had varied accuracy (100% vs. 32.1%) in identifying non-wear. Conclusion: The exclusive use of inclinometry for sedentary behavior and non-wear determination is not recommended.