The Republic (Ancient Greek: Πολιτεία, romanized: Politeia; Latin: De Republica) is a Socratic dialogue authored by Plato around 375 BC, concerning justice (dikaiosúnē), the order and character of the just city-state, and the just man. It is Plato's best-known work, and one of the world's most influential works of philosophy and political theory, both intellectually and historically.

In the dialogue, Socrates discusses with various Athenians and foreigners the meaning of justice and whether the just man is happier than the unjust man. He considers the natures of existing regimes and then proposes a series of hypothetical cities in comparison, culminating in Kallipolis (Καλλίπολις), a utopian city-state ruled by a class of philosopher-kings. They also discuss ageing, love, theory of forms, the immortality of the soul, and the role of the philosopher and of poetry in society. The dialogue's setting seems to be the time of the Peloponnesian War.

The Republic is generally placed in the middle period of Plato's dialogues. However, the distinction of the middle dialogues from the early dialogues is not as clear as the distinction of the late dialogues from all the others. Nonetheless, Ritter, Arnim, and Baron—each adopting separate methodologies—agree that the Republic was well distinguished, along with Parmenides, Phaedrus and Theaetetus.

The first book of the Republic, which shares many features with earlier dialogues, is thought to have originally been written as a separate work, with the remaining books were conjoined to it, perhaps with modifications to the original of the first book.

Socrates and Glaucon visit Athens' port, Piraeus, when they are invited to join Polemarchus for a dinner and festival. At Polemarchus' house, Socrates encounters Polemarchus' father, Cephalus.

In his first philosophical conversation with the group members, Socrates asks the aged Cephalus, "is life painful at that age, or what report do you make of it?" Cephalus answers by saying that many are unhappy about old age because they miss their youth, but he finds that "old age brings us profound repose and freedom from this and other passions. When the appetites have abated, and their force is diminished, the description of Sophocles is perfectly realized. It is like being delivered from a multitude of furious masters." The repose gives him time to dedicate himself to sacrifices and justice so that he is prepared for the afterlife.

The discussion then turns to the definition of justice. Cephalus says that justice is honesty and returning to people that which they are owed. Socrates refutes this by arguing that it would not be just to honestly return a borrowed knife to a man who has since gone mad and would harm himself with it.

Polemarchus argues that justice is giving what is appropriate to others — to do good to one's friends and bad to his enemies. Socrates refutes this in four parts, concluding that harming anyone, even one's enemies, in itself creates injustice, and thus cannot appear just.