Cotys I or Kotys I (Ancient Greek: Κότυς, Kotys) was a king of the Odrysians in Thrace from 384 BC to his murder in 360 BC. He was known to have been born during the reign of Seuthes I, based on ancient sources and date of birth estimates for Cotys, his daughter who married the Athenian general Iphicrates, and her son Menestheus. According to Harpokration, he reigned for 24 years, which places his accession in 384 BC. Although his origins are actually unknown, An Athenian inscription dated to 330 BC, which honors Reboulas, brother of Cotys and son of king Seuthes. As the ordinal of Seuthes is not mentioned, it was unclear, however, which of the preceding kings named Seuthes is meant by the inscription. While scholars originally believed Seuthes II to be the father of Cotys I, now it is known that Seuthes I was his father, as Seuthes II was only 7 years old at the time of Seuthes I's abdication in 411 BC.
In 390 BC the Athenian general Iphicrates joined his colleague Thrasybulus in Thrace, helping arrange peace between the Thracian rulers Amadocus I and Seuthes II, and making both Athenian allies. Iphicrates remained for about two years, and married the daughter of Cotys in opulent circumstances. Cotys' precise standing at the time remains obscure, but he is tentatively identified as a brother of Hebryzelmis, who was the Odrysian king in Thrace in 386/385 BC, when he was honored in an Athenian decree. Shortly afterwards, in 384 BC, Cotys I became king, perhaps at first in the interior parts of Thrace. With the help of Iphicrates, Cotys succeeded in establishing himself as the heir of Hebryzelmis, and eliminating the opposition of his rivals, possibly including Amadocus II and Saratocus. Archaeological finds of items inscribed for Cotys to the north and west suggest expansion and contacts in that direction that have escaped the narrative sources. The name of Cotys (in one instance described as a son of the god Apollon) appears several times on objects from the Rogozen Treasure, although the precise manner in which it came to be buried in northwestern Bulgaria, presumably in the territory of the Triballi, remains unclear.
As Cotys tried to enlarge his kingdom, including towards the Aegean and the Chersonese, his actions led to increasing tensions with Athens. In the early 370s BC, the Second Athenian Confederacy was founded with a number of neighboring cities and islands joining the Confederacy as a safeguard against the threat from Cotys, among others.
In 375 BC Cotys probably supported Hales, leader of Triballi, a powerful Thracian tribe in northwestern Thrace, in their attack on the city of Abdera. According to Diodorus, the city was saved only after the intervention of the Athenian general, Chabrias, whose forces then garrisoned the city. In 367 BC Ariobarzanes, the Persian satrap of Phrygia, occupied Sestos. Following Ariobarzanes' revolt against the Persian king Artaxerxes II in 365 BC, Cotys apparently opposed Ariobarzanes and his ally, Athens, but his actions are ambivalent and unclear. The same year Cotys welcomed the return of his son-in-law, the dismissed Athenian general Iphicrates, and a new ally, the mercenary commander Charidemus, who married another daughter of Cotys. The Athenians under their general Timotheus were able to gain Sestos and Krithote. Soon after, in 363 or 362 BC, Cotys sent Miltokythes into the Thracian Chersonese to take Sestos. Miltokythes besieged the city and starved it into surrender, then attacked Krithote. Athenian generals sent to relieve the attacked towns were unsuccessful.