Giant congenital melanocytic nevus (GCMN) is a rare birthmark disorder that reportedly affects 1 in 20,000-500,000 live births. Here, we present a case of GCMN in a 1-day-old newborn that covered the entire abdomen, reaching the thigh and chest, and laterally toward the backward aspect of the trunk to involve the entire back and buttocks. We discuss the diagnostic and treatment approach.