Objectives This study was performed to ultrasonographically assess the masseter muscle thicknesses of individuals with disk displacement with reduction. Methods The thickness of the masseter muscle in 100 patients (28 male, 72 female; average age 34 years) who presented with one-sided temporomandibular joint pain and a clicking sound and were diagnosed with disk displacement with reduction was measured in relaxation and at maximum contraction using ultrasonography. Results In the contracted position, the difference in the masseter muscle thickness between the healthy side and the side with disk displacement with reduction was statistically significant (p < 0.05). However, no statistically significant difference was found between the two sides in the relaxed position. Conclusions Obvious ultrasonographic changes of the masseter muscle were found in patients with temporomandibular disk displacement with reduction, and these changes might be related to unilateral chewing. Individuals with anterior disc displacement probably chew unilaterally because of pain and clicking.