Background/objectives <p>The reliability of isometric strength measurements in morbidly obese individuals is a critical area of research due to the connection with functional performance. The objectives of this study were to assess the test–retest reliability of upper-limb isometric strength using functional electromechanical dynamometer (FEMD), and to investigate the correlation between upper-limb isometric strength and handgrip strength measurements.</p> Methods <p>Twenty-one morbidly obese women and men (age: 43.53 ± 9.99&#xa0;years, height: 157.22 ± 7.12&#xa0;cm, weight: 105.09 ± 12.31&#xa0;kg, Body Mass Index: 43 ± 5.4&#xa0;kg/m²) participated in the study. All participants were evaluated for anthropometric parameters, handgrip strength and upper-limb isometric strength.</p> Results <p>The FEMD provides high reliability for assessing upper-limb isometric strength in both dominant [ICC of 0.89 (95% CI 0.81–0.96) and a CV of 5.98%] and non-dominant [ICC of 0.92 (95% CI 0.82–0.97) and a CV of 5.59%] arms, and moderate concurrent validity was found between upper-limb isometric strength and handgrip strength measurements for the non-dominant limb (<i>r</i> = 0.51, <i>p</i> = 0.021), while no significant correlation was found on the dominant side (<i>r</i> = 0.41, <i>p</i> = 0.071).</p> Conclusions <p>Our results suggest that FEMD is a reliable tool for assessing upper-limb isometric strength in morbidly obese individuals and establishing a standardized protocol. Moderate concurrent validity was found between upper-limb isometric strength and handgrip strength. The FEMD could be introduced as a training and assessment tool in individuals with morbid obesity, allowing the systematization and control of training load components during functional movements.</p>