Autosomal recessive osteopetrosis is also known as infantile malignant osteopetrosis (IMO). The clinical course is often serious and if left untreated, it is fatal in the 1st year of life. Diagnosis is challenging and often delayed or misdiagnosed. Herein, we present an infant girl who was diagnosed with IMO during evaluations for her hypotonicity and thrombocytopenia. A novel mutation of the chloride voltage-gated channel 7 (CLCN7) gene was also reported. A 10-day-old female patient was referred to our hospital for evaluation of hypotonicity. Her physical examination was normal, other than hypotonicity. Laboratory analysis revealed thrombocytopenia and hypocalcemia. In the progress, while she was followed in outpatient clinic, hepatosplenomegaly was detected at the age of 3 months. IMO was suspected with the findings of hepatosplenomegaly, cytopenia, hypocalcemia, difficulty of obtaining bone marrow, peripheral smear findings, and hearing loss. The X-ray of the bones was consistent with IMO. A novel pathogenic homozygous c.1504>T (p.Arg502Trp) mutation in CLCN7 gene was revealed. IMO is a rare disorder and it is important to differentiate this entity for better clinical outcome. The presence of neurological and hematological findings, organomegaly, hearing loss, and vision disorders must attract attention to IMO.