Utnapishtim is the ark builder in the Epic of Gilgamesh, the ancient flood story compared with Noah’s Ark.
In the flood episode of the Epic of Gilgamesh, the god Ea warns Utnapishtim to build a boat and preserve living creatures, craftsmen, and provisions. Utnapishtim later describes the vessel and the flood to Gilgamesh, who seeks the secret of eternal life.
The Mesopotamian account shares several broad motifs with Genesis, including divine warning, a large vessel, survival of a flood, birds sent out to test the waters, and a sacrifice afterward. It is not the same text as Genesis and has different characters, gods, and details.
Utnapishtim is sometimes called Atrahasis in related Mesopotamian traditions. Those names belong to interconnected but distinct literary traditions, so the specific answer for the Epic of Gilgamesh is Utnapishtim.