This study aimed to investigate the effects of obesity on the internal mechanical work, and its influence on the total mechanical work, energy cost, and mechanical efficiency in obese and nonobese adults while walking at different speeds. Body composition and anthropometrical characteristics were obtained for eleven obese [O; 39.9 ± 7.9 yr; body mass index (BMI): 43.0 ± 4.2 kg/m ² ] and thirteen lean adults (L; 29.6 ± 5.7 yr; BMI: 22.0 ± 1.5 kg/m ² ). Participants walked at five speeds (0.56, 0.83, 1.11, 1.39, 1.67 m/s) while oxygen consumption was measured to obtain net energy cost of walking (NC _w ). A motion analysis system and instrumented treadmill were combined to obtain external (W _ext ), internal (W _int ), and total (W _tot ) mechanical work, and pendular energy recovery. Mechanical efficiency was calculated as the ratio of W _tot to NC _w . Relative NC _w (per unit body mass) was significantly higher in O than L (P ≤ 0.001). Relative W _ext was significantly lower in O compared with L (P = 0.002), whereas no significant difference was found in relative W _int (P = 0.16) and W _tot (P = 0.6). Recovery was significantly higher (P ≤ 0.001), while mechanical efficiency was significantly lower in O than in L (P ≤ 0.001). These results suggest that individuals with obesity class III have similar mass-normalized W _int and W _tot compared with their lean counterparts, along with a higher relative NC _w . Consequently, the efficiency of walking was reduced in this population. These results suggest that mass-normalized W _int is unaffected by obesity and is not responsible for the higher relative NC _w and lower efficiency of walking in these individuals.NEW & NOTEWORTHY It has been suggested that internal mechanical work (i.e., the work required to move the limbs with respect to the center of mass, W _int ) may be responsible for the higher net cost of walking in obese adults, but this variable has not yet been studied in individuals with obesity. The main finding of the present study is that individuals with class III obesity exhibit a similar amount of mass-normalized W _int to that of adults with a normal body weight, suggesting that body mass-relative W _int is not affected by obesity and is not responsible for the higher energy cost and the lower efficiency of walking in this population.