We present a case of double-chambered right ventricle diagnosed during preparation for colonoscopy due to gastrointestinal bleeding in a 16-year-old, mentally disabled boy with Williams syndrome. The patient was previously diagnosed with ventricular septal defect and mild pulmonary stenosis. Echocardiography performed under general anesthesia revealed hypertrophied muscular bundles in the right ventricle with the maximum gradient of 100 mmHg, causing severe outflow obstruction. This type of defect is extremely rare in patients with Williams syndrome, with only one case, which was diagnosed during invasive angiocardiography, described in world literature. A successful total surgical correction was performed based on echocardiography data.