Here, we report an infantile case of ventricular septal defect and restrictive cardiomyopathy caused by a de novo mutation of the cardiac troponin T gene. Initially diagnosed with a perimembranous ventricular septal defect in the newborn period, we observed the first signs of restrictive cardiomyopathy at the age of 18 months. The patient was listed for cardiac transplantation at the age of 8 years despite optimal medical treatment. The finding of a de novo mutation in TNNI3 (R204H) enabled a genetic diagnosis and counselling. We suggest that the previously reported overlap of functional and morphologic phenotypes in sarcomeric genes may also be a feature of TNNI3 mutations.