Individuals with Class III dentofacial deformities exhibit morphological changes and myofunctional adaptations, and an accurate diagnosis is essential for treatment planning. The purpose of this study was to evaluate the electrical activity of the masseter, temporal, and suprahyoid muscles in subjects with Class III dentofacial deformities, assessing electromyographic characteristics using functional tests. The research group consisted of 20 subjects with Class III dentofacial deformities and indications for surgical-orthodontic treatment. The control group consisted of 10 individuals presenting a good maxilla and mandible relationship. Electrical activities of the masseter and temporal were recorded during isometric contraction and maximal isometric contraction. The suprahyoid and masseter were assessed during maximal mouth opening and swallowing of saliva. Isometric contraction and maximal isometric contraction analysis showed less potential in the research group, with a significant difference regarding the masseter muscle in isometric contraction. In maximal isometric contraction, an increase in electrical activity in both groups was observed; it remained lower for the research group, but with no significant difference. With regard to symmetry, significant differences among the groups were observed only for the left masseter in the research group, as well as during isometric contraction. There was no significant difference found regarding maximal mouth opening. During swallowing of saliva, there was a high percentage of abnormal electromyographic tracings for both groups. This finding was not expected for the control group. The study found some changes in muscular activity in Class III dentofacial deformities, characterized by lower potential in the masseter muscle during isometric contractions. No other peculiarities were found that could contrast either group.