Background: Many studies have shown that poor posture may lead to myofascial pain dysfunction syndrome (MPDS) of the trunk. Forward head posture is one of the most common forms of poor posture, and is related to neck pain. Hence, the aim of this study was to estimate the influence of body posture on MPDS of the temporomandibular joint (TMJ). Methods: Twenty-six otherwise healthy adults with MPDS of the TMJ were enrolled in this study. Clinical examination was performed at three time-points: prior to the treatment, at the 2-week follow-up, and at the 4-week follow-up. The subjects received a muscle relaxant drug and adjustment of body posture. Variables such as visual analogue scale (VAS) score, shoulder angle (SA), cervical spine angle-coronal (CSA-c), and cervical spine angle-sagittal (CSA-s) were evaluated at each stage. Generalized estimating equations (GEE) were used to control for non-independence among observations. Results: There were significant improvements in VAS score (p < 0.001), SA (p < 0.001), CSA-c (p < 0.001), and CSA-s (p < 0.001) after treatment compared to the corresponding values obtained before treatment. Conclusions: Correcting body posture in conjunction with use of a muscle relaxant had an extremely therapeutic effect on patients with MPDS of TMJ. The improvement of the angle of sagittal cervical spine may have reduced the pain score, and we speculate that it is important to correct forward head posture when treating patients with MPDS of TMJ.