Objective <p>This study aimed to evaluate sleep bruxism (SB) in patients with obstructive sleep apnea (OSA) by assessing the thickness and stiffness of the masseter and temporalis muscles using ultrasonography and shear wave elastography (SWE).</p> Methods <p>This cross-sectional study included 68 patients diagnosed with OSA. The presence of SB was assessed using clinical criteria. Demographic and clinical variables—including age, gender, body mass index (BMI), apnea–hypopnea index (AHI), Epworth Sleepiness Scale (ESS), and maximum mouth opening—were recorded. Bilateral ultrasonography and SWE measurements of the masseter and temporalis muscles were performed at rest and during clenching. Comparisons were made between bruxist and non-bruxist patients, as well as between those with mild–moderate and severe OSA. Statistical analyses were conducted using SPSS v30.0. Independent-samples t-tests and chi-square tests were applied; <i>p</i> ≤ 0.05 was considered significant.</p> Results <p>SB was observed in 51.5% of patients with OSA. Compared to those without SB, patients with SB had significantly lower right masseter muscle thickness at rest (<i>p</i> = 0.036) and during clenching (<i>p</i> = 0.019). In the analysis based on OSA severity, patients with severe OSA showed significantly greater thickness in the right masseter muscle at rest (<i>p</i> = 0.009) and during clenching (<i>p</i> = 0.022), as well as in the left temporalis muscle at rest (<i>p</i> = 0.017) and during clenching (<i>p</i> = 0.026). However, no significant differences were found in muscle stiffness values across any of the groups (<i>p</i> &gt; 0.05).</p> Conclusions <p>These findings suggest that while muscle thickness may reflect structural adaptations in OSA and SB, stiffness measurements via SWE did not differentiate between groups, highlighting the need for further research on its clinical utility.</p>