The Peloponnesian League (/pɛləpəˈniːʃən/) was an alliance of ancient Greek city-states, dominated by Sparta and centred on the Peloponnese, which lasted from c. 550 to 366 BC. It is known mainly for being one of the two rivals in the Peloponnesian War (431–404 BC), against the Delian League, which was dominated by Athens.

The Peloponnesian League is the modern name given to the Spartan system of alliances, but it is inaccurate because there were members outside the Peloponnese, and it was not really a league. The ancient name of the League was "the Lacedemonians and their allies". This is misleading as well, because Sparta could have allies outside of the Peloponnesian League.

In its early history, Sparta expanded by conquering Laconia and Messenia and reducing their population into slavery (as helots), but the subjugation of Tegea on its northern border failed at the battle of the Fetters. Following this defeat, Sparta abandoned its military conquests and adopted a diplomatic strategy, known as the "bones policy", by appropriating the relics of mythical heroes worshipped in the Peloponnese, starting with Orestes, the son of Agamemnon, whose bones were transferred from Tegea to Sparta. This new diplomacy was likely sponsored by Chilon, ephor c. 556, who therefore enabled Sparta to present itself as the natural successor of the mythical Achaean kingdom of Agamemnon as described by Homer. Tegea then signed an alliance treaty with Sparta, which became the starting point of the subsequent Peloponnesian League.

Tegea was pushed towards Sparta by its fear of Argos, its eastern neighbour. For the same reason, all the other neighbours of Argos rapidly concluded treaties with Sparta on the Tegean model: Mantinea, Phlieus, Corinth, Epidaurus and the other cities of Argolis. They were followed by Elis, the large city of the western Peloponnesus, and all the Arcadian communities of central Peloponnesus. By 540s, Sparta had concluded alliances with all the Peloponnesian cities, apart from Argos and Achaean cities on the northern shore.

There was no collective treaty between all the members of the League. As hegemon (leader of the League), Sparta concluded a separate treaty with each member, which therefore entered the League upon its conclusion. Each member swore the same oath with Sparta: "to have the same friends and enemies as the Spartans, and to follow them withersoever they may lead". League members were consequently not bound together, only to Sparta, and could even wage war on each other. However, in 378 a League decision forbade internal wars if the League was operating an army outside of the Peloponnese, but perhaps this disposition had already been in place from much earlier and was a part of the constitution of the League. L. H. Jefery summarises the constitution of the League as "a circle centred on Sparta, with the spokes of a wheel but not necessarily with the added cross-links of a web."

The League treaties contained defensive obligations: Sparta had to assist an ally attacked by a non-League member, and conversely the allies had to help Sparta in case of an attack. The famous Spartan fear of the helots is shown by a special clause providing that allies had to assist Sparta in case of a slave revolt and must not offer citizenship to Messenians, because the Arcadians assisted the latter during the Spartan conquest of Messenia. This clause was activated in the 460s during the Third Messenian War. The treaties between Sparta and the allies were also permanent, with a clause forbidding secession. Several secessions did occur, but as a result of a breach of a treaty. Seceding members usually pointed out a breach of the treaties from Sparta to leave. The procedure to admit new members is not known. Sparta could either decide alone, or request the approval from their allies in the subsequent League congress.

Another reason for the allies to remain within the League, despite their loss of autonomy, was the support of oligarchy by Sparta. The oligarchs that ruled most of the League members could rely on Sparta to retain their status in their city. Moreover, many of them had friendship ties with Spartan citizens, or even the kings. The Spartan king Agesilaus II (r. c. 400 – c. 360) was especially known for his guest friendships (xenia) among his allies. Thanks to these friendships, leading oligarchs could send their sons to the agoge, the Spartan education system, where they became trophimoi xenoi, and further developed their attachment to Sparta.

A major change in the organisation of the League took place c. 506, when the Spartan king Cleomenes I attempted to capture Athens and place at its head his friend Isagoras as tyrant or as member of an oligarchy. A full army of the League was called and marched on Athens, but the Corinthians returned home when they discovered the purpose of the expedition, also encouraged by the other king Demaratus, who opposed Cleomenes. The campaign therefore failed, and as a result Sparta had to concede the creation of a congress of the League, where members could vote on war and peace. It means that before that time, Sparta could call its allies at will without informing them of the purposes of the war.