Introduction: Metabolic adaptations to maximal physical exercise in people with obesity (PwO) are scarcely described. This cross-sectional study evaluates the metabolic response to exercise via the respiratory exchange ratio (RER) in PwO and different degrees of glycemic control. Methods: Eighty-five PwO (body mass index 46.0 [39.0–54.0] kg/m<sup>2</sup>), that is, 32 normoglycemic (Ob-N), 25 prediabetic (Ob-preDM), and 28 diabetic (Ob-T2DM) subjects and 18 healthy subjects performed an incremental, maximal cardiopulmonary exercise test. The RER was measured at rest (RERrest) and at peak exercise (RERpeak). Results: RERpeak was significantly higher in healthy subjects than that in PwO. Among those, RERpeak was significantly higher in Ob-N than that in Ob-preDM and Ob-T2DM (1.20 [1.15–1.27] vs. 1.18 [1.10–1.22] p = 0.04 and vs. 1.14 [1.10–1.18] p < 0.001, respectively). Accordingly, ΔRER (RERpeak-RERrest) was lower in Ob-preDM and Ob-T2DM than that in Ob-N (0.32 [0.26–0.39] p = 0.04 and 0.29 [0.24–0.36] p < 0.001 vs. 0.38 [0.32–0.43], respectively), while no significant difference was found in ΔRER between Ob-preDM and Ob-T2DM and not even between Ob-N and healthy subjects. Moreover, ΔRER in PwO correlated with glucose area under curve (p = 0.002). Conclusions: PwO demonstrate restricted metabolic response during maximal exercise. Particularly, those with prediabetes already show metabolic inflexibility during exercise, similarly to those with type 2 diabetes. These findings also suggest a potential role of cardiopulmonary exercise testing in detecting early metabolic alterations in PwO.