Achaia (Ancient Greek: Ἀχαΐα), sometimes spelled Achaea, was a province of the Roman Empire, consisting of the Peloponnese, Attica, Boeotia, Euboea, the Cyclades and parts of Phthiotis, Aetolia and Phocis. In the north, it bordered on the provinces of Epirus vetus and Macedonia. The region was annexed by the Roman Republic in 146 BC following the sack of Corinth by the Roman general Lucius Mummius, who was awarded the surname "Achaicus" ('conqueror of Achaia'). Initially part of the Roman province of Macedonia, it was made into a separate province by Augustus.

Achaia was a senatorial province, thus free from military men and legions, and one of the most prestigious and sought-after provinces for senators to govern. Athens was the primary center of education for the imperial elite, rivaled only by Alexandria, and one of the most important cities in the Empire. Achaia was among the most prosperous and peaceful parts of the Roman world until late antiquity, when it first suffered from barbarian invasions. The province remained prosperous and highly urbanized, however, as attested in the 6th-century Synecdemus.

The Slavic invasions of the 7th century led to widespread destruction, with much of the population fleeing to fortified cities, the Aegean islands and Italy, while some Slavic tribes settled the interior. The territories of Achaia remaining in Byzantine hands were grouped into the theme of Hellas.

In 150–148 BC the Romans fought the Fourth Macedonian War, after which they annexed Macedon, formerly the largest and most powerful state in mainland Greece. In 146 BC the Achaean League initiated the Achaean War against Rome. The contemporary historian Polybius blames the demagogues of the cities of the Achaean League for encouraging a rash decision and inciting a suicidal war. The League was quickly defeated by Lucius Mummius and its main city, Corinth, was destroyed. After the war, the Romans annexed mainland Greece. A group of ten commissioners "put down democracies" in the Greek cities (Pausanias) through a programme of "constitutional restructuring" which involved the introduction of property qualifications for participating in civic politics, temporarily abolished the Achaean, Boeotian, Locrian, and Phocaean Leagues, and levied tribute on the individual cities. However, the cities remained mostly self-governing. Athens and Sparta, which had not participated in the war remained autonomous and free. It is disputed whether Greece became part of the Roman province Macedonia or was left unincorporated. Interventions by the governor of Macedonia in Greek affairs are attested, but also the dispatch of separate legates direct from Rome. Roman governance over the following century remained "rather ad hoc."

In the Dyme Affair of 144 BC, a faction in the city of Dyme passed laws "contrary to the type of government granted by the Romans," staged a revolution, and destroyed their town hall and official records. At the request of the Dymaean town councillors, Quintus Fabius Maximus issued a ruling, sentencing the revolutionaries to death. An inscription recording judicial decisions made in the Greek city of Demetrias in the mid-second century BC says that the judgements were made in accordance with local law and "the edicts and judgements of the Romans", indicating that Roman law was already considered to apply to the region only a few years after the Achaean War.

In the following decades, many Greek communities sought to establish treaty relationships of "friendship and alliance" with Rome, apparently finding this preferable to free status. Treaties are attested, mostly by inscriptions, with Epidaurus and Troezen in the late second century BC, Astypalaea in 105 BC, Thyrium in 94 BC. The cities probably sought these treaties as a way of safeguarding their territory from their larger neighbours. Rome was increasingly called upon by the Greek communities to arbitrate in disputes between them, instead of seeking inter-state arbitration as had been common in the Hellenistic period. In these disputes, "friends and allies" of the Romans were usually favoured.

The First Mithridatic War (89–85 BC) was fought in Attica and Boeotia, two regions which were to become part of the province of Achaia. In 89 BC, Mithradates VI Eupator, king of Pontus, seized the Roman Province of Asia (in western Anatolia). Mithridates then sent Archelaus (his leading military commander) to southern Greece, where he established Aristion as a tyrant in Athens. The Roman consul Lucius Cornelius Sulla landed in Epirus (in western Greece) and marched on Athens. He marched through Boeotia on his way to Attica. Sulla besieged Athens and Piraeus in 87-86 BC and then sacked Athens and destroyed Piraeus. He then defeated Archelaus at the Battle of Chaeronea and the Battle of Orchomenus, both fought in Boeotia in 86 BC. Roman rule was preserved. Following the war, Sulla pardoned the Greek cities that had followed Mithridates and restored the legal systems that had been given to them by the Romans previously.

As the part of the Roman East closest to Italy, Greece was a central theatre of the civil wars of the Late Republic. The war between Julius Caesar and Pompey the Great culminated in Caesar's victory at the Battle of Pharsalus in Thessaly in 48 BC. In 46 BC, southern Greece was separated out from Macedonia as a separate province for the first time by Julius Caesar, who placed it under a proconsul, but this was reversed at some point after his assassination in 44 BC. Caesar also ordered the refoundation of Corinth, abandoned since 146 BC, as a Roman colony. Caesar's assassins, led by Marcus Junius Brutus and Gaius Cassius Longinus, based themselves in Greece during the Liberators' civil war, until their defeat by Octavian and Mark Antony of the Second Triumvirate at the Battle of Philippi in 42 BC. After the battle, the Second Triumvirate assigned Greece along with the rest of the East to Mark Antony, who remained in control of it until his defeat by Octavian at the Battle of Actium in 31 BC.