Background Rectus sheath hematoma is a condition where blood is accumulated within the sheath of the rectus abdominis muscle. It is caused by the rupture of either the superior or inferior epigastric artery or from a direct tear of the rectus muscle itself. This condition is usually self-limiting and most of the time, the hematoma is small, non-expanding and can be reabsorbed spontaneously over time. However, certain underlying pathology may lead to the expansion of the hematoma or even a recurrence, and without any medical treatment or intervention, this condition can even be fatal. Due to its rarity, only a few published cases of fatality associated with rectus sheath hematoma are reported worldwide. Case presentation We present a case of non-traumatic fatal rectus sheath hematoma in the presence of underlying lung infection. The deceased complained of abdominal pain 2 days prior to death with a history of prolonged cough. The autopsy revealed the presence of a hematoma within the rectus sheath of the left lower quadrant of the abdomen extending down into the left pelvis and a multitude of small yellow-tan firm lesions scattered throughout the parenchyma of the right lung, suggestive of a lung infection which was later confirmed histologically. Conclusions The case serves to increase awareness of fatal rectus sheath hematoma as a cause of death given its rarity in forensic medicine practice. The importance of a thorough post-mortem examination is the key to noticing the fatal hematoma since its enclosed location within the rectus sheath may be obscured to the eye of an inexperienced prosector.