Eru Ilúvatar is a deity in J.R.R. Tolkien's Middle-earth legendarium. He is introduced in The Silmarillion as the creator of all existence (called Eä in Elvish). In Tolkien's invented language of Elvish, Eru means "The One", or "He that is Alone" and Ilúvatar signifies "Father of All". The names appear in Tolkien both in isolation and paired (Eru Ilúvatar).

Eru is a supreme being, analogous to God. Eru is transcendent, and completely outside of and beyond the world. He first created a group of angelic beings, called in Elvish the Ainur, and these holy spirits were co-actors in the creation of the universe through a holy music and chanting called the "Music of the Ainur", or Ainulindalë in Elvish.

Eru alone can create independent life or reality by giving it the Flame Imperishable. All beings not created directly by Eru, (e.g. Dwarves, Ents, Eagles), still need to be accepted by Eru to become more than mere puppets of their creator. Melkor desired the Flame Imperishable and long sought for it in vain, but he could only twist that which had already been given life.