By the late Neolithic period, the basins of the Ch'orokh, Q'orolis-ts'q'ali, and Cholok rivers, in what is now Ajaria, were already inhabited by humans. In Kobuleti region (at Ispaani), excavations in peat strata uncovered a settlement dating to the third millennium B.C. The basin of the Ch'orokh, which has played a significant role in the history of Georgian culture, politics, and socioeconomic life, was an ancient center of mining and metallurgy (copper, bronze, and iron). During the second millennium B.C. a metallurgical industry developed, exploiting local sources of ore, and in the first quarter of the first millennium B.C. the ancient Colchian iron industry (centered in the basins of the Ch'orokh and Cholok-Ochkhamur rivers) played a significant role in the development of the socioeconomic life of the region. At the time of the flourishing of Colchian civilization, this region became the primary Colchian center for the production of iron. The tradition of mining and metallurgy continued through succeeding periods in the history of the region and has been partially maintained up to the present.
The tribes inhabiting the territory of Ajaria during the seventh to fifth centuries B.C. achieved a high level of cultural development. Clear evidence is provided by the excavation of a Colchian grave at Pich'varni (Ch'orokhi Basin, near Kobuleti). The archaeological materials found at the Pich'varni site provide a valuable source for studying the trade, economic, and cultural relations between the Colchian civilization and the antique world during the classical period. West Georgian (Zan) tribes inhabiting the territory of Ajaria, Guria, and Samegrelo (Mingrelia), along with other ethnic groups that lived along the eastern coast of the Black Sea in ancient times, united to form the Colchian Empire. As Byzantine Greek and Georgian manuscripts attest, this empire long maintained close trade and economic relations with the peoples of Asia Minor and the Mediterranean. Old Georgian historical documents mention Ajaria only sporadically. The first written attestation is the Ashkharatsuyts (Geography), composed by an anonymous Armenian author of the seventh century, in which the historical-ethnographic regions of Georgia are enumerated: K'larjeti, Art'aani, Shavsheti, Javakheti, Samtskhe, and so on, including Ajaria. According to the eleventh-century Georgian chronicler Leonti Mroveli, during the fourth to third centuries B.C. Georgia was subdivided into military-administrative districts (in Georgian: saeristaoni, "duchies"), one of which was Ajaria. (Written tradition ascribes the division of the country into saeristaoni to King Parnavaz.)
In the year 65 B.C. the Roman Empire expanded its influence into the territory of the confederated West Georgian tribes but was rebuffed by Colchian and Iberian (East Georgian) tribes. In the fourth century A.D. , because of the hegemony exercised by the Lazes (one of the Colchian tribes), the Laz Empire (also known as Egrisi) was established. Its southern part included the territory of Ajaria, with its major settlements—Kobuleti, Tsikhisdziri, Batumi, and Gonio. During this period Christians propagated their faith in Ajaria. Georgian written sources attribute the initiative for this process to the apostle Andrew the First-Called. In the early feudal period Persia and Byzantium became rivals in the struggle for the possession of western Georgia. At about that same time the fortified city of Petra (Justinianopolis) was erected in Lazica by the order of Justinian. The city of Petra (contemporary Tsikhisdziri) had an advantageous strategic and geographical location on the Black Sea coast and played a crucial role in the political and cultural-economic life of western Georgia at that time. It was the seat of a bishopric of Byzantine orientation: in the tenth century the episcopal see located there formed part of the Laz eparchy, which was subordinate to the patriarchate of Constantinople.
In the seventh century the Arabs conquered eastern Georgia (Iberia). Many people, seeking to save themselves from the enemy, resettled in western Georgia, which had not fallen into the invaders' hands. The process of the Iberization of the Colchian tribes was already under way before the time of Christ, but in this region the formation of new ethnic groups (Ajarians, Gurians) was not completed until the early feudal period, during which the Arabic conquest served as a catalyst for the melding of western Colchian autochthons with immigrants from eastern Georgia. With the formation of the Ajarían and Gurian ethnographic groups, the Black Sea coastal lands belonging to the Colchians (Mingrelians and Lazes) were divided into four territories: Samegrelo, Guria, Ajaria, and Lazeti. The inhabitants of Samegrelo and Lazeti spoke, and still speak, a language related to Georgian called Zan, which has two dialects (Mingrelian and Laz), whereas the population of Guria and Ajaria spoke Georgian dialects.
Several feudal principalities gradually formed in southwestern Georgia: Guria, Ajaria, Sp'eri, Samtskhe, Javakheti, and others, which subsequently were integrated into a more powerful feudal domain called T'ao-K'larjeti. In the ninth to tenth centuries T'ao-K'larjeti became one of the major feudal centers of Georgia, and within T'ao-K'larjeti Ajaria played a not insignificant role, making a substantial contribution to the development of the centralized polity and culture of Georgia. In the tenth century Ajaria received the status of a saeristao and formed a part of united Georgia. In the Middle Ages power in Ajaria was in the hands of the Abuserisdze clan, who were eristavebi (equivalent to dukes) named by the king of united Georgia. A particularly noteworthy representative of this family was Tbel Abuserisdze, a well-known scholar and author of several original works on astronomy and philosophy.
After repulsing the invading Seljuk Turks in the twelfth to thirteenth centuries, Georgia became one of the major powers of the Near East. At this time in Ajaria a distinctive Christian culture came to fruition, and various secular and religious edifices—arched bridges, monasteries, and churches, mostly of the basilica type—were erected. The brief existence of the centralized Georgian state ended with the destructive Mongol invasions of the thirteenth to fourteenth centuries, followed by the division of the country into small feudal principalities. In 1453 Byzantium fell before the attacks of the Ottoman Empire, and thus Georgia was deprived of its primary Christian ally. In the sixteenth century the Ottomans began systematic raids upon the southern borders of Georgia, and step by step conquered T'ao-K'larjeti, Lazeti, etc. In the seventeenth century the Ottomans seized the territory of Samtskhe-Javakheti (Meskheti) and undertook a lengthy struggle for Ajaria, culminating in its total incorporation into the Ottoman Empire.
With Russia's breaking of the Georgyevsk Treaty, signed by Russia and Georgia—in 1783, Georgia—in the form of two provinces, Tbilisi and Kutaisi—was forcibly incorporated into the Russian Empire. Following the Russo-Turkish War of 1877-1878, on 25 August 1878, Russia returned the territory of Ajaria to Georgia. Under the system of czarist colonial administration, Ajaria was renamed the Batumi District ( okrug ) . In 1918, after the Russian Revolution, Georgia proclaimed its independence and, as the Georgian Democratic Republic, was recognized by many foreign governments. The agreement concerning Georgian independence, signed by Soviet Russia and democratic Georgia in 1920, was violated shortly afterward: in 1921 Red Army divisions occupied Georgia and established the Georgian Socialist Republic. Following consultations with Kemalist Turkey, the Moscow government handed over a large part of the territory of southwestern Georgia to Turkey in 1921, and in Ajaria the government artificially created an autonomous republic (Ajarían ASSR) within the borders of the Georgian SSR. After the establishment of Soviet power, the historical territory of Georgia was significantly reduced as the Soviet government gave large parts of the republic's territory to Turkey and Russia. In all, Georgia lost about 20,000 square kilometers; the present-day Georgian Republic consists of 69,500 square kilometers, of which Ajaria occupies 2,900 square kilometers.