The Odrysian kingdom (/oʊˈdrɪʒən/; Ancient Greek: Βασίλειον Ὀδρυσῶν), also known as Odrysia, was an ancient Thracian state that thrived between the early 5th century BC and the early 3rd / late 1st century BC. Located in present-day Bulgaria, southeastern Romania (Northern Dobruja), northern Greece and European Turkey, it was a tribal amalgam dominated by the Odrysians that was the first large political entity to develop in the eastern Balkans.

The Odrysian kingdom was founded by king Teres I, who was exploiting the collapse of the Persian presence in Europe due to the failed invasion of Greece in 480–79. Teres and his son Sitalces pursued a policy of expansion, making the kingdom one of the most powerful of its time. Throughout much of its early history it remained an ally of Athens and even joined the Peloponnesian War on its side. By 400 BC the state showed first signs of fatigue, although the skilled Cotys I initiated a brief renaissance that lasted until his murder in 360 BC.

Afterwards the kingdom disintegrated: southern and central Thrace were divided among three Odrysian kings, while the northeast came under the dominion of the kingdom of the Getae. The three Odrysian kingdoms were eventually conquered by the rising kingdom of Macedon under Philip II in 340 BC. A much smaller Odrysian state was revived in around 330 BC by Seuthes III, who founded a new capital named Seuthopolis that functioned until the second quarter of the 3rd century BC. After that there is little conclusive evidence for the persistence of an Odrysian state, with the exception of a dubious Odrysian king fighting in the Third Macedonian War named Cotys. The Odrysian kingdom was attacked by the Roman Republic in the late 1st century BC, when the Odrysian heartlands eventually became known as the Sapaean kingdom, a client state of the Roman Republic, which was finally abolished and converted into a Roman province of Thracia in 45-46 AD.

Since the ancient Thracians lacked an indigenous writing tradition, the most important sources for the reconstruction of their history are archaeological remains, coins as well as accounts of ancient Greek historians. Said historians considered the Thracians to be a numerous people and their country, Thrace, to be of barely comprehensible size, so large that Andron of Halicarnassus (4th century BC) thought of it as a continent of its own. While the boundaries of Thrace fluctuated throughout history, Thrace can be divided in a northern and a southern half, which were also culturally different. The border between the two halves has been identified as the Haemus Mountains or the Danube slightly further north. Southern Thrace covered the fertile valley between the Haemus and the Rhodopes, the Strandzha and the shores of the Propontis and the Aegean and Black Seas. The western boundary was marked by the Strymon and the upper Morava. Northern Thrace was defined by the Danube, the Carpathians and the adjacent western tip of the Pontic–Caspian steppe, thus enclosing parts of the territory now comprising modern Romania, Moldova, Serbia and Ukraine. Thrace also extended into what is now northwestern Turkey both west and east (Bithynia, Mysia) of the Propontis.

Thracians had already settled in the 2nd millennium BCE, and were featured in the epics of Homer. Occasional references to them appear in the following centuries, although it was not until the 5th century when Greek literature developed an interest in discussing non-Greeks more extensively. In the 7th and 6th centuries, much of the Thracian coast was settled by Greek colonists who founded numerous towns, like Thasos, Byzantion or Odessos. The political history of the Thracian tribes of this age is virtually unknown, although it is recorded that in the late 6th century, Athenian settlers interacted with a "king of Thrace" (and possible predecessor of the Odrysian kings?) residing north of the Chersonese peninsula. The absence of imported artefacts confirms that inland Thrace north of the Rhodopes remained largely isolated from the Aegean trade until the late 6th century.

In around 513 BC, an army of the mighty Persian dynasty of the Achaemenids crossed the Bosphorus, after already having subdued the Thracians of Bithynia thirty years earlier. King Darius I's goal was a punitive expedition against the Scythians at the northern shores of the Black Sea. Most eastern Thracian tribes submitted peacefully, except of the Getae, who were defeated. More expeditions under the generals Megabazus and Mardonius as well as king Xerxes I followed, even though they only managed to secure the Aegean coast.

It seems most likely that the Achaemenids did not establish a satrapy (provincial administration) in Thrace, even though the historian Herodotus claimed that the subdued regions had to pay taxes. As a matter of fact, there is no evidence for important administrative centers. Instead, Persian authority was merely exercised through a couple of garrisoned forts, most importantly those of Doriskos and Eion. Hence, the vast majority of Thrace remained unaffected by the Persian presence. After the failed invasion of Greece in 480-79, the Persian foothold in Europe collapsed. By around 450, Persian authority in Europe, including Thrace, had vanished entirely.

Although the Persian presence in Thrace was short-lived, it probably stimulated trade and first state formations among the Thracians. Mintings of Thracian coins started around 500 and may be an indicator for a variety of early tribal kingdoms. It has been suggested that the Odrysian kingdom might have had its origins in this period, even though the name of the Odrysians is notably absent from the numismatic evidence. The Odrysians eventually stepped into the light of history in the aftermath of the Persian failure in Greece, when they were mentioned by Herodotus, but without any further details. The Odrysians had their core territory in the valleys of the Hebros river and its tributaries Tonzos and Arda. Like other Thracian polities, the Odrysian tribal kingdom attempted to fill the vacuum left by the Persian retreat.