Assyria was a major ancient Mesopotamian civilization that existed as a city-state from the 21st century BC to the 14th century BC and eventually expanded into an empire from the 14th century BC to the 7th century BC.

Spanning from the early Bronze Age to the late Iron Age, modern historians typically divide ancient Assyrian history into the Early Assyrian (c. 2600–2025 BC), Old Assyrian (c. 2025–1364 BC), Middle Assyrian (c. 1363–912 BC), Neo-Assyrian (911–609 BC), and post-imperial (609 BC–c. AD 240) periods, based on political events and gradual changes in language. Assur, the first Assyrian capital, was founded c. 2600 BC, but there is no evidence that the city was independent until the collapse of the Third Dynasty of Ur, in the 21st century BC, when a line of independent kings starting with Puzur-Ashur I began ruling the city. Centered in the Assyrian heartland in northern Mesopotamia, Assyrian power fluctuated over time. The city underwent several periods of foreign rule or domination before Assyria rose under Ashur-uballit I in the early 14th century BC as the Middle Assyrian Empire. In the Middle and Neo-Assyrian periods, Assyria was one of the two major Mesopotamian kingdoms, alongside Babylonia in the south, and at times became the dominant power in the ancient Near East. Assyria was at its strongest in the Neo-Assyrian period, when the Assyrian army was the strongest military power in the world and the Assyrians ruled the largest empire then yet assembled in world history, spanning from parts of modern-day Iran in the east to Egypt in the west.

The Neo-Assyrian Empire fell in the late 7th century BC, conquered by a coalition of the Babylonians, who had lived under Assyrian rule for about a century, and the Medes. Though the core urban territory of Assyria was extensively devastated in the Medo-Babylonian conquest of the Assyrian Empire and the succeeding Neo-Babylonian Empire invested few resources in rebuilding it, ancient Assyrian culture and traditions continued to survive for centuries throughout the post-imperial period. Assyria experienced a recovery under the Seleucid and Parthian empires, though it declined again under the Sasanian Empire, which sacked numerous cities and semi-independent Assyrian territories in the region, including Assur itself. The remaining Assyrian people, who have survived in northern Mesopotamia to modern times, were gradually Christianized from the 1st century AD onward. Ancient Mesopotamian religion persisted at Assur until its final sack in the 3rd century AD, and at certain other holdouts for centuries thereafter.

The triumph of ancient Assyria can be attributed not only to its vigorous warrior-monarchs but also to its adeptness in efficiently assimilating and governing conquered territories using inventive and advanced administrative mechanisms. The developments in warfare and governance introduced by ancient Assyria continued to be employed by subsequent empires and states for centuries. Ancient Assyria also left a legacy of great cultural significance, particularly through the Neo-Assyrian Empire, making a prominent impression in later Assyrian, Greco-Roman, and Hebrew literary and religious tradition.

In the Old Assyrian period, when Assyria was merely a city-state centered on the city of Assur, the state was typically referred to as ālu Aššur ("city of Ashur"). From the time of its rise as a territorial state in the 14th century BC and onward, Assyria was referred to in official documents as māt Aššur ("land of Ashur"), marking its shift to being a regional polity. The first attested use of the term māt Aššur is during the reign of Ashur-uballit I (c. 1363–1328 BC), who was the first king of the Middle Assyrian Empire. Both ālu Aššur and māt Aššur derive from the name of the Assyrian national deity Ashur. Ashur probably originated in the Early Assyrian period as a deified personification of Assur itself. In the Old Assyrian period the deity was considered the formal king of Assur; the actual rulers only used the style Išši'ak ("governor"). From the time of Assyria's rise as a territorial state, Ashur began to be regarded as an embodiment of the entire land ruled by the Assyrian kings.

The modern name "Assyria" is of Greek origin, derived from Ασσυρία (Assuría). The term's first attested use is during the time of the ancient Greek historian Herodotus (5th century BC). The Greeks called the Levant "Syria" and Mesopotamia "Assyria", even though the local population, both at that time and well into the later Christian period, used both terms interchangeably to refer to the entire region. It is not known whether the Greeks began referring to Mesopotamia as "Assyria" because they equated the region with the Assyrian Empire, long fallen by the time the term is first attested, or because they named the region after the people who lived there, the Assyrians. Because the term is so "similar to Syria", scholars have been examining since the 17th century whether the two terms are connected. And because, in sources predating the Greek ones, the shortened form "Syria" is attested as a synonym for Assyria, notably in Luwian and Aramaic texts from the time of the Neo-Assyrian Empire, modern scholars overwhelmingly support the conclusion that the names are connected.

Both "Assyria" and the contraction, "Syria," are ultimately derived from the Akkadian Aššur. Following the decline of the Neo-Assyrian Empire, the subsequent empires that held dominion over the Assyrian lands adopted distinct appellations for the region, with a significant portion of these names also being rooted in Aššur. The Achaemenid Empire referred to Assyria as Aθūrā ("Athura"). The Sasanian Empire inexplicably referred to Lower Mesopotamia as Asoristan ("land of the Assyrians"), though the northern province of Nōdšīragān, which included much of the old Assyrian heartland, was also sometimes called Atūria or Āthōr. In Syriac, Assyria was and is referred to as ʾĀthor.

Agricultural villages in the region that would later become Assyria are known to have existed by the time of the Hassuna culture, c. 6300–5800 BC. Though the sites of some nearby cities that would later be incorporated into the Assyrian heartland, such as Nineveh, are known to have been inhabited since the Neolithic, the earliest archaeological evidence from Assur dates to the Early Dynastic Period, c. 2600 BC. During this time, the surrounding region was already relatively urbanized. There is no evidence that early Assur was an independent settlement, and it might not have been called Assur at all initially, but rather Baltil or Baltila, used in later times to refer to the city's oldest portion.