Congenital lactose intolerance is a rare autosomal recessive disorder characterized by complete or near-complete absence of lactase enzyme activity from birth. Affected infants present with severe osmotic diarrhea soon after initiation of lactose-containing feeds. Early diagnosis is essential to prevent dehydration, electrolyte imbalance, and growth failure. We report a 6.5-month-old male infant presenting with chronic watery diarrhea since birth, who showed dramatic clinical improvement following initiation of a lactose-free diet. Reappearance of symptoms after reintroduction of lactose further supported the diagnosis. This case highlights the importance of considering congenital lactose intolerance in infants with persistent diarrhea since birth and emphasizes the diagnostic and therapeutic role of lactose withdrawal.