When eaten, pufferfish is indeed toxic enough to kill a human. The tetrodotoxin found in the fish's organs, including its liver, is strong enough to bring about respiratory failure. There is no known antidote to this toxin, and the pufferfish species that do carry it carry enough to kill multiple people.
Despite, or perhaps because of, the risk of consuming it, pufferfish is consumed raw as a form of sushi known as fugu. The fish's flesh isn't poisonous enough to do harm, though chefs who do not prepare it with enough care can cause the poison from the organs to spread to the flesh, which is the main risk posed by eating fugu.