The Sacred Band of Thebes (Ancient Greek: Ἱερὸς Λόχος τῶν Θηβῶν, Hieròs Lóchos tôn Thēbôn) was an elite heavy infantry of select soldiers, allegedly consisting of 150 pairs of male couples, 300 men total, organized by age that formed the elite force of the Theban army in the 4th century BC, it was first organised under commander Gorgidas in 378 BC and later Pelopidas, and played a crucial role in the Battle of Leuctra. It was annihilated by Philip II of Macedon and young Alexander the Great in the Battle of Chaeronea in 338 BC.

The earliest surviving record of the Sacred Band by name was in 324 BC, in the oration Against Demosthenes by the Athenian logographer Dinarchus. He mentions the Sacred Band as being led by the general Pelopidas and, alongside Epaminondas who commanded the army of Thebes (Boeotia), were responsible for the defeat of the Spartans at the decisive Battle of Leuctra (371 BC).

And if there were only some way of contriving that a state or an army should be made up of lovers and their beloved, they would be the very best governors of their own city, abstaining from all dishonour, and emulating one another in honour; and when fighting at each other's side, although a mere handful, they would overcome the world. For what lover would not choose rather to be seen by all mankind than by his beloved, either when abandoning his post or throwing away his arms? He would be ready to die a thousand deaths rather than endure this. Or who would desert his beloved or fail him in the hour of danger?

Plutarch (46–120 AD), a native of the village of Chaeronea, is the source of the most substantial surviving account of the Sacred Band. He records that the Sacred Band was originally formed by the boeotarch Gorgidas, shortly after the expulsion of the Spartan garrison occupying the Theban citadel of Cadmea. The 2nd century AD Macedonian author Polyaenus in his Stratagems in War also records Gorgidas as the founder of the Sacred Band. However, Dio Chrysostom (c. 40–120 AD), Hieronymus of Rhodes (c. 290–230 BC), and Athenaeus of Naucratis (c. 200 AD) credit Epaminondas instead.

The exact date of the Sacred Band's creation, and whether it was created before or after the Symposium of Plato (c. 424–347 BC) and the similarly titled Symposium by his rival Xenophon (c. 430–354 BC), has also long been debated. The generally accepted date of the Sacred Band's creation is between 379 and 378 BC. Prior to this, there were references to elite Theban forces also numbering 300. Herodotus (c.484–425 BC) and Thucydides (c. 460–395 BC) both record an elite force of 300 Thebans allied with the Persians, who were annihilated by Athenians in the Battle of Plataea (479 BC). Herodotus describes them as "the first and the finest" (πρῶτοι καὶ ἄριστοι) among Thebans. Diodorus also records 300 picked men (ἄνδρες ἐπίλεκτοι) present in the Battle of Delium (424 BC), composed of heníochoi (ἡνίοχοι, "charioteers") and parabátai (παραβάται, "those who walk beside"). Though none of these mention the Sacred Band by name, these may have referred to the Sacred Band or at least its precursors. Historian John Kinloch Anderson believes that the Sacred Band was indeed present at Delium, and that Gorgidas did not establish it, but merely reformed it.

In the old debate surrounding Xenophon's and Plato's works, the Sacred Band has figured prominently as a possible way of dating which of the two wrote their version of Symposium first. Xenophon's Socrates in his Symposium disapprovingly mentions the practice of placing lovers beside each other in battle in the city-states of Thebes and Elis, arguing that while the practice was acceptable to them, it was shameful for Athenians. Both Plato and Xenophon were Athenians. According to the British classical scholar Sir Kenneth Dover, this was a clear allusion to the Sacred Band, reflecting Xenophon's contemporary, albeit anachronistic, awareness of the Theban practice, as the dramatic date of the work itself is c. 421 BC.

However, it is the speech of the character Phaedrus in Plato's Symposium referring to an "army of lovers" that is most famously connected with the Sacred Band; even though it does not technically refer to the Sacred Band, since the army referred to is hypothetical. Dover argues Plato wrote his Symposium first since Plato's Phaedrus uses language that implies that the organization does not yet exist. He acknowledges, however, that Plato may have simply put the hypothesis in the mouth of Phaedrus according to the supposed earlier dramatic date of the work (c. 416 BC). It only shows that Plato was more mindful of his chronology in his Symposium than Xenophon, and proves that he was actually quite aware of the Sacred Band in his time.

According to Plutarch, the 300 hand-picked men were chosen by Gorgidas purely for ability and merit, regardless of social class. It was composed of 150 male couples, each pair consisting of an older erastes (ἐραστής, 'lover') and a younger eromenos (ἐρώμενος, 'beloved'). Athenaeus of Naucratis also records the Sacred Band as being composed of "lovers and their favorites, thus indicating the dignity of the god Eros in that they embrace a glorious death in preference to a dishonorable and reprehensible life", while Polyaenus describes the Sacred Band as being composed of men "devoted to each other by mutual obligations of love". The origin of the "sacred" appellation of the Sacred Band is unexplained by Dinarchus and other historians. But Plutarch claims that it was due to an exchange of sacred vows between lover and beloved at the shrine of Iolaus (one of the lovers of Heracles) at Thebes. He also tangentially mentions Plato's characterization of the lover as a "friend inspired of God".