A standing army is a permanent, often professional, army. It is composed of full-time soldiers who may be either career soldiers or conscripts. It differs from army reserves, who are enrolled for the long term, but activated only during wars or natural disasters, and temporary armies, which are raised from the civilian population only during a war or threat of war, and disbanded once the war or threat is over. Standing armies tend to be better equipped, better trained, and better prepared for emergencies, defensive deterrence, and particularly, wars. The term dates from approximately 1600, although the phenomenon it describes is much older.

Sargon of Akkad, the founder of the Akkadian Empire, is believed to have formed the first standing professional army. Tiglath-Pileser III of Assyria (ruled 745–727 BC) created Assyria's first standing army. Tiglath-Pileser III disbanded militias and instead paid professional soldiers for their services. His army was composed largely of Assyrian soldiers but was supplemented with foreign mercenaries and vassal states. The standing army he created was the most sophisticated administrative and economic institution of its time, and was the engine of Assyrian economy which capitalized on warfare.

Cyrus the Great formed the first professional army of Persia. The composition of the army varied and developed in the course of time. The empire's great armies were, like the empire itself, very diverse. Its standing army was composed of Persians (the bravest people of empire according to Herodotus) and Medes. This standing army, which may have been reviewed every year by the king or his representative, is called kāra in the inscriptions. At the heart of this army was its elite guard, The 10,000 Immortals. Herodotus describes that if any of these guardsmen drops out owing to death or disease, a substitute is immediately supplied and the number again filled. Thousands of these 10,000 guardsmen composed the royal bodyguards in the palace, their insignia were golden apples or pomegranates at the butts of their spears (accordingly they are named “apple-bearers” by Heraclides Cumaeus).

In ancient Greece, the city-states' (poleis) armies were essentially drafted citizen militias. The exception was in ancient Sparta, which had a standing army that trained year-round (and not only in summertime). Through the 5th century BC, they comprised the only professional soldiers in ancient Greece, aside from hired mercenaries. However, the Spartan army commonly consisted of helots (serfs), who considerably outnumbered the Spartiates, as well as numerous allies of Sparta.

Philip II of Macedon instituted the first true professional Hellenic army, with soldiers and cavalrymen paid for their service year-round, rather than a militia of men who mostly farmed the land for subsistence and occasionally mustered for campaigns.

The Western Zhou maintained a standing army, enabling them to effectively control other city states and spread their influence. Unlike the Western Zhou, the Eastern Zhou initially did not have a standing army. Instead they drafted militias from around 150 city states. While the Eastern Zhao did not initially maintain a standing army, the state of Jin became the first to do so in 678 BCE. The first professional army in China was established by the Qin dynasty in 221 BCE, which ushered Imperial China. Under the Qin dynasty, wars were fought by trained vocational soldiers instead of relying on temporary soldiers.

In Ancient India, warfare was first attested during the Vedic period. However, warfare was primarily waged between various clans and kingdoms solely by the kshatriya class during times of conflict. True standing armies in India developed under the Mahajanapadas, which relied on paid professional soldiers year round. The most prominent of the Mahajanapadas was the Kingdom of Magadha. It is accepted that the first standing army of India was created in Maghada by the ruler Bimbisara.

Under the reign of Augustus, the first Roman emperor, a standing professional army of the Roman Empire was gradually instituted, with regularized pay. This professional force of legionaries was expensive to maintain, but supported the authority of the empire, not only as combat troops but also as provincial police forces, engineers, and guards. Legionaries were citizen volunteers entitled to a discharge bounty upon 25 years of honorable service; supplementing the legions were the auxilia, auxiliary forces composed of non-citizens in the provinces who typically earned citizenship as a reward for service.