Orchomenus (Ancient Greek: Ὀρχομενός Orchomenos), the setting for many early Greek myths, is best known today as a rich archaeological site in Boeotia, Greece, that was inhabited from the Neolithic through the Hellenistic periods. It is often referred to as "Minyan Orchomenus", to distinguish it from a later city of the same name in Arcadia.

According to the founding myth of Orchomenos, its royal dynasty was established by the Minyans, who had followed their eponymous leader Minyas from coastal Thessaly to settle the site.

In the Bronze Age, during the fourteenth and thirteenth centuries BC, Orchomenos became a rich and important centre of civilisation in Mycenaean Greece and a rival to Thebes. The palace with its frescoed walls and the great beehive tomb show the power of Orchomenos in Mycenaean Greece. A massive hydraulic undertaking drained the marshes of Lake Kopaïs, making it a rich agricultural area. Orchomenos is mentioned among the Achaean cities sending ships to engage in the Trojan War in Homer's "Catalogue of Ships" in the Iliad: together with Aspledon, they contributed thirty ships and their complement of men. The wealth brought in via the agricultural land under its control probably explains its Homeric epithet "rich in gold" (πολύχρυσος).

Many histories of Mycenaean Greece contend that Orchomenos was, like Pylos, one of the cities destroyed during the turbulent period referred to as the Bronze Age Collapse. Ancient Greek legends attribute the destruction to a war with the neighboring power of Thebes. However, Jesse Millek recently challenged this assumption, claiming it is based on misreading archeological evidence that the site was simply abandoned during the same time period. Bryan Feuer commented that archeological information on Mycenaean Orchomenos is fragmentary in part because much of its remains "have been destroyed by erosion or subsequent rebuilding".

Orchomenos seems to have been one of the city-states that joined the Calaurian maritime League in the seventh century BC. Although their rivals Thebes confirmed their supremacy by the end of the century reflected by inscriptions, Orchomenos joined the Theban-led Boeotian League in c. 600 BC.

Classical Orchomenos was known for its sanctuary of the Charites or Graces, the oldest in the city (the 9th century Byzantine monastery church of Panagia Skripou probably occupies the same spot). Here the Charites had their earliest veneration, in legend instituted by Eteocles; musical and poetical agonistic games, the Charitesia, were held in their honour, in the theatre that was discovered in 1972. The Agrionia, a festival of the god Dionysus, involved the ritual pursuit of women by a man representing Dionysus. Orchomenos struck its coinage from the mid-sixth century.

In 480–479 BC, the Orchomenians joined their neighbouring rivals the Thebans to turn back the invading forces of Xerxes in the Greco-Persian Wars. In mid-century, Orchomenos sheltered the oligarchic exiles who freed Boeotia from Athenian control. In the fourth century the traditional rivalry with Thebes made Orchomenos an ally of Agesilaus II and Sparta against Thebes, in 395 and again in 394 BC. The Theban revenge after their defeat of Sparta in the Battle of Leuctra (371 BC) was delayed by the tolerant policies of Epaminondas: the Boeotian League sacked Orchomenos in 364 BC. Although the Phocians rebuilt the city in 355 BC, the Thebans destroyed it again in 349.

The broad plain between Orchomenos and the acropolis of Chaeronea witnessed two battles of major importance in Classical antiquity. In 338 BC, after a whirlwind march south into central Greece, Philip II of Macedon defeated Thebes and Athens on the plain of Chaironeia during the first Battle of Chaeronea, establishing Macedonian supremacy over the city-states, and demonstrated the prowess of Philip's young son Alexander the Great. During Alexander's campaign against Thebes in 335 BC, Orchomenos took the side of the Macedonians. In recompense, Philip and Alexander rebuilt Orchomenos, when the theatre and the fortification walls, visible today, were constructed.