The fictional island where J.R.R. Tolkien’s Númenóreans lived was called Númenor.
Númenor was an island kingdom in Tolkien’s legendarium, located in the Great Sea between Middle-earth and Aman. The Valar granted it to the Men who had aided the Elves during the First Age. Númenóreans were granted unusually long lifespans, though they remained mortal.
Over many generations, Númenor’s people became increasingly powerful and developed a deep fear of death. Their king Ar-Pharazôn eventually challenged the authority of the Valar and sailed west with a great army. As a consequence of this rebellion, Númenor was destroyed and submerged. Elendil and his sons escaped, founding kingdoms in Middle-earth.
Númenor is sometimes confused with Valinor, the realm of the Valar, or Beleriand, a region of Middle-earth in the First Age. Tolkien also called Númenor Westernesse.