Background <p>The efficacy of arthrocentesis in temporomandibular joint (TMJ) disorders is well established, but its effect on articular capsular width is unclear. The purpose of this study was to evaluate changes in capsular width associated with TMJ arthrocentesis using ultrasonography.</p> Methods <p>This prospective cohort study included individuals presenting with temporomandibular disorders (TMD) to Van Yuzuncu Yil University. Inclusion criteria were systemically healthy adults who provided informed consent and had intra-articular TMD persisting for ≥ 6 months despite conservative management. Statistical analyses included Mann-Whitney U test, Wilcoxon signed-rank test, and Friedman test for group comparisons, with significance at <i>p</i> &lt; 0.05. Spearman correlation analysis was performed to assess associations between changes in capsular width and clinical outcomes.</p> Results <p>The study sample consisted of 27 subjects (16 females, 59.3%) with a mean age of 41.6 ± 13.0 years. Ultrasonographic analysis revealed a significant reduction in capsular width on the affected side over time (<i>p</i> &lt; 0.001), whereas no significant change was observed on the non-affected side (<i>p</i> = 0.4). The mean capsular width on the affected side decreased from 2.73 ± 0.29&#xa0;mm at baseline (T0, pre-treatment) to 2.46 ± 0.30&#xa0;mm at three months after arthrocentesis (T1) and 2.29 ± 0.33&#xa0;mm at six months after arthrocentesis (T2). Between-group comparisons showed no significant difference at T0 (<i>p</i> = 0.3) or at T1 (<i>p</i> = 0.1), but at T2, the affected side demonstrated a significantly smaller capsular width compared with the non-affected side (<i>p</i> &lt; 0.001).</p> Conclusions <p>The findings suggest that arthrocentesis was associated with a reduction in capsular width on the treated side, concurrent with improvements in pain and mandibular function.</p> Trial registration <p>ClinicalTrials.gov (NCT07553078), registered on April 15, 2026. Retrospectively registered.</p>