Objective <p>The purpose of current study was to evaluate changes in masseter muscle thickness (MMT) and elasticity in patients with symptomatic apical periodontitis (SAP) using ultrasonography before and after endodontic treatment.</p> Methods <p>This prospective controlled clinical study inclusive 30 patients diagnosed with SAP and 30 systemically and orally healthy individuals. MMT was assessed using B-mode ultrasonography, and tissue stiffness was measured with shear wave elastography (SWE). Measurements were obtained bilaterally at rest and during maximum clenching. In the case group, the second measurement was performed 1-month after the completion of root canal treatment and without the observation of symptoms. Data were investigated using independent and paired samples t-tests, statistical importance level of <i>P</i> &lt; 0.05 was accepted.</p> Results <p>Intra-observer agreement was good (κ = 0.83). At baseline, the right MMT was importantly lower in the case group compared with the control group, both at rest and during clenching (<i>p</i> = 0.003 and <i>p</i> = 0.020, respectively). Similarly, right-sided SWE values obtained during clenching were significantly reduced in the case group (<i>p</i> = 0.003), whereas no significant difference was found for resting SWE measurements.</p> <p>On the left side, resting measurements did not diverge importantly between groups. However, during clenching, both muscle thickness and SWE values were significantly lower in the case group (<i>p</i> = 0.024 and <i>p</i> = 0.001).</p> Conclusion <p>SAP may cause alterations in MMT and stiffness. The improvement observed in ultrasonographic parameters after root canal treatment suggests that these changes may represent a reversible functional adaptation to acute periapical inflammation.</p> Clinical relevance <p>The clinical significance of this study focuses on elucidating how SAP and associated pain can lead to secondary adaptations in the masseter muscle, such as protective co-contraction or muscle protection.</p>