Cotys I is King of the Odrysian Kingdom in 383 BC to 359 BC.
In order to make his position stronger Cotys married his daughter to the Athenian general Iphicrates who soon became the second person in command after the king. Cotys starts to pursue an active foreign policy, which used renting Greek army and commanders.
In 376-375 BC Cotys used the leader of powerful Thracian tribe of Triballi in war over the town of Abdera and placed him under control. The Triballi, rebelled against his kingdom. One of the reasons for this revolt was that the Triballi were unable to get luxurious goods and other items from the south. Cotys stopped the rebellion by rebuilding the Greek city of Pistiros.
Cotys violate the union with Athens. He went to war with the Athenians for the possession of the Thracian Chersonese, which aims to turn Odrysian Kingdom the marine force and the first Balkan factor. In 362 BC he controlled peninsula except the cities Elayos, Sestos and Kritote. In 361 BC Athens organized a rebellion against Cotys, led by his treasurer Miltokythes. Yet Iphicrates, with the help of Charidemus, bribed the Athenian military and naval commanders to suppress the rebellion. In 361 BC, Charidemus returned to Athens with a treaty from Cotys, proclaiming him an ally. Cotys had successful retained his kingdom.
By 359 BC, Cotys controlled the whole Chersonese peninsula. This circumstance and contacts that Cotys I established with new Macedonian King Philip II, brought to his murder, organized by the intervention of Athens.
Cotys I policy and his actions in Thracian Hersones deep disrupting the Second Athenian sea league and became the main factor that prepare and makes it easier Philip II victory over Athens. After his death Odrysian kingdom collapsed.