Atenism, also known as the Aten religion, the Amarna religion, and the Amarna heresy, was a religion in ancient Egypt. It was founded by Akhenaten, a pharaoh who ruled the New Kingdom under the Eighteenth Dynasty. The religion is described as monotheistic or monolatristic, although some Egyptologists argue that it was actually henotheistic. Atenism was centered on the cult of Aten, a god depicted as the disc of the Sun. Aten was originally an aspect of Ra, Egypt's traditional solar deity, though he was later asserted by Akhenaten as being the supreme of all deities.

In the 14th century BC, Atenism was Egypt's state religion for around 20 years, and Akhenaten met the worship of other gods with persecution; he closed many traditional temples, instead commissioning the construction of Atenist temples, and also suppressed religious traditionalists. However, subsequent pharaohs toppled the movement in the aftermath of Akhenaten's death, thereby restoring Egyptian civilization's traditional polytheistic religion. Large-scale efforts were then undertaken to remove from Egypt and Egyptian records any presence or mention of Akhenaten, Atenist temples, and assertions of a uniquely supreme god.

The traditional form of ancient Egyptian religion was polytheistic, encompassing hundreds of deities. The Egyptians defined these gods by describing their relationships and interactions with one another. As the Egyptologist Jan Assmann put it, "a deity was not conceivable without reference to other deities". The relationships between deities were often expressed through mythological imagery. For instance, the movement of the sun through the sky was often envisioned as a divine king, the sun god Ra, sailing through the heavens in a barque, accompanied by subordinate deities who served as his crew and defended the barque against the forces of chaos. Deities might also be linked with, or syncretized, with each other to represent the overlap in their roles. Ra was often syncretized with the sky god Horus as Ra-Horakhty, representing the sun at the horizon, or with the creator god Atum, reflecting Egyptian creation myths, in which the creation of the world coincided with the rising of the sun.

The gods were worshipped in temples in cities across Egypt, and the chief deity of a city's main temple was considered the city's patron deity. Making offerings to the gods, and therefore managing the temples where the offerings took place, was one of the core duties of the pharaoh, the king of Egypt. Over the course of Egyptian history, the more significant temples became major landholders, their activities supported by vast estates worked by thousands of people. These estates were ultimately under royal control, and a king could have a major effect on the economy by revising the distribution of temple resources.

Over the history of ancient Egypt, deities shifted in their prominence relative to each other. During the Old Kingdom (c. 2686–2181 BC), Ra was the foremost deity. The ideology surrounding Ra, in which the sun god was the father of the pharaoh, became integral to Egyptian kingship, and many Old Kingdom rulers incorporated Ra's name into their own. Whereas other deities were believed to manifest themselves in cult statues in enclosed sanctuaries within temples, Ra's temples and shrines centered on open-air courts, where the focus of worship was either the sun itself or a sacred stone known as the benben. The original temple of this type was at Ra's cult center of Heliopolis in Lower Egypt. Several rulers in the Fifth Dynasty in the late Old Kingdom built their own sun temples, centering on massive obelisks that may have been modeled on the benben of Heliopolis. Many Egyptologists therefore see the Fifth Dynasty as the peak of Ra's worship.

Ra remained one of the most important deities in the pantheon even after the Old Kingdom, and many other major deities developed syncretistic links with him. During the Middle Kingdom (c. 2055–1650 BC), rulers from Thebes in Upper Egypt elevated that city's patron deity to national prominence, syncretizing him with Ra as Amun-Ra. Amun was considered a mysterious, unknowable deity, but the theology of Amun-Ra made Ra the visible face of Amun's hidden power. Amun assumed even more importance early in the New Kingdom (c. 1550–1070 BC), when the rulers of the Eighteenth Dynasty (c. 1550–1292 BC) conquered large areas of Nubia, Canaan, and Syria and poured much of the wealth from these conquests into temples, especially Amun's main temple at Karnak in Thebes. Thutmose III, for instance, made many additions to Karnak that emphasized Amun's link with the sun, and Thebes itself became known as the "Heliopolis of Upper Egypt".

Some texts from the early New Kingdom, before the emergence of Atenism, characterize Amun-Ra in a novel way, which Assmann dubbed the "new solar theology". In these texts, primarily hymns found in tombs of the period, the sun god's actions in the daily movement through the sky are characterized as his alone, ignoring the deities who were traditionally said to assist him. The hymns stressed that the sun's light gave life to all living things, and in the multiethnic empire of the New Kingdom, even foreign peoples—traditionally considered agents of chaos in Egyptian ideology—could be thought of as the subjects of the sun god's beneficent rule.

At the same time, another term for the sun god was growing increasingly prominent. The word jtn, or "aten", originally referred to a circle or a disk and, in the Coffin Texts near the beginning of the Middle Kingdom, began to be used specifically for the disk of the sun. It was originally treated as an inanimate object, in contrast with the sun god, who was thought of as sitting within or shining through the Aten. But the Aten increasingly came to be treated as a deity, and in the early New Kingdom solar hymns, it was increasingly closely connected with the sun god, whether Ra or Amun.