Purpose <p>Molar-incisor hypomineralisation (MIH) impacts children’s oral health and quality of life, but research on the stomatognathic system of unilateral cases, remains limited. This study evaluated the effects of unilateral MIH on the stomatognathic system in children, focusing on postural and masticatory activities and occlusal force contacts.</p> Methods <p>In this cross-sectional observational study, 28 children (mean age: 8.21&#xa0;years) were divided into two groups: without MIH (CG) and with unilateral MIH (MIHG). Electromyography (EMG) evaluated the activity of masseter and temporalis muscles during various jaw postures and habitual chewing. Occlusal force contacts were measured using T-SCAN®. Comparisons were made between groups and sides. Statistical analysis employed the Shapiro–Wilk test, Wilcoxon signed-rank test, and paired t-test, with a significance level of 5%.</p> Results <p>EMG data demonstrated increased muscle hyperactivity in MIHG compared to CG at rest for the left temporalis muscle on the contralateral side (p &lt; .001), during left laterality for the right temporalis muscle on the ipsilateral side (p = 0.04), and habitual chewing of soft food for the left temporalis muscle on the contralateral side (p = 0.02). Greater voluntary contraction was observed on the unaffected side in MIHG for the temporalis muscle at rest (p = 0.01) and the masseter muscle during right laterality (p = 0.02). No significant differences were found in occlusal force measurements between groups or sides.</p> Conclusions <p>This study indicates that unilateral MIH is associated with increased muscle hyperactivity in the contralateral temporalis and masseter muscles, without significant differences in occlusal force.</p>