<p>The primary aim of this study was to examine the validity of energy expenditure (EE) estimated via branched chain modeling (BCM) during treadmill walking with a weighted vest. Thirteen sedentary adults with overweight/mild obesity [M = 11, F = 2; 29.0 (5.5) yr; maximal oxygen uptake (V̇O_2max): 27.4 (2.0) mL·kg⁻¹·min⁻¹_; body mass index (BMI) = 29.6 (3.0)] were recruited. Participants completed a ramp incremental and steady-state walking (4.0, 4.5, 5.0, and 5.5&#xa0;km·h⁻¹) whilst wearing weighted vest (10% body mass; VEST) and unweighted control (CON). EE was estimated via indirect calorimetry (EE_IC) and BCM (EE_BCM), using a combined accelerometry/heart rate (HR) sensor (ActiHeart 5), and cardiorespiratory responses were determined from gas analysis. Limits of agreement (LoA) and concordance correlation coefficient (<i>CCC</i>) were used to assess validity of EE_BCM. Repeated-measures ANOVA (condition_*velocity) was used to examine physiological responses. Significant main effects of VEST included elevated oxygen consumption (V̇O₂; ml.kg⁻¹.min⁻¹;<i>p</i> &lt; 0.001), ventilation (<i>p</i> &lt; 0.001), respiratory exchange ratio (RER; <i>p</i> &lt; 0.001), HR (<i>p</i> &lt; 0.001), and both EE_IC (<i>p</i> &lt; 0.001) and EE_BCM (<i>p</i> &lt; 0.001). There was a strong correlation between EE_IC and EE_BCM in both CON (<i>CCC</i> = 0.80) and VEST (<i>CCC</i> = 0.82). Mean bias (95% LoA) between EE_IC and EE_BCM was − 6.7 (-71.5,58.2) J·kg⁻¹·min⁻¹ (CON) and 3.6 (-50.3,57.6) J·kg⁻¹·min⁻¹ (VEST). Adults with overweight/mild obesity demonstrated an increase in EE during treadmill walking with a weighted vest. BCM was sensitive to this increase and also to treadmill velocity., However the agreement in estimated EE between indirect calorimetry and BCM was marginal for both CON and VEST conditions, and thus not interchangeable.</p>