* In the traditional Greek myth: Polyphemus was blinded by Odysseus, the protagonist of Homer's Odyssey. Odysseus and his crew got trapped in Polyphemus' cave and were threatened to be eaten. Odysseus got Polyphemus drunk, then blinded him with a sharpened stake.
* In some retellings: Polyphemus' death is attributed to other characters. Some stories say he was killed by Hercules or by Hermes, who was sent by Zeus to punish Polyphemus for his cruelty.
* It's unclear if Polyphemus actually died: The Odyssey only describes Polyphemus being blinded. It's possible he lived on, although his blindness would have made his life difficult.
Therefore, while Odysseus is the most commonly accepted answer, it's important to acknowledge that other interpretations exist.