Purpose: The aim of this study was to examine changes in the electromyographic activity, thickness, width, and hardness of the masseter muscle from before to after orthognathic surgery. Material and Methods: The study included 15 patients with Class III dentofacial deformities who were treated with combined orthodontic and orthognathic surgery. Fifteen individuals with normal occlusion and no signs or symptoms of temporomandibular joint dysfunction were used as controls. All records were obtained bilaterally in the study group before surgery (T1), at 3 months after surgery (T2), and in the control group (CG) while at rest and in maximum voluntary contraction (MVC). Results: There was no difference in resting masseter muscle activity between T1, T2, and CG. Resting thickness and width of the masseter muscle did not differ significantly between T1 and T2. MVC masseter muscle activity and thickness increased significantly and width decreased significantly from T1 to T2 but did not reach CG values. Muscle hardness increased from T1 to T2. Conclusions: The authors' findings indicate that despite improved muscle activity and dimensions, postoperative 3 months is still early period for adaptation of the masseter muscles to the new occlusion and skeletal morphology.