Roman Italy is the period of ancient Italian history going from the founding and rise of Rome to the decline and fall of the Western Roman Empire; the Latin name of the Italian peninsula in this period was Italia (continued to be used in the Italian language). According to Roman mythology, Italy was the ancestral home of Aeneas, being the homeland of the Trojans progenitor, Dardanus; Aeneas, instructed by Jupiter, moved to Italy after the fall of Troy, and his descendants, Romulus and Remus, were the founders of Rome. Aside from the legendary accounts, Rome was an Italic city-state that changed its form of government from Kingdom (ruled, between 753 BC and 509 BC, by seven kings) to Republic, and then grew within the context of a peninsula dominated by the Gauls, Ligures, Veneti, Camunni and Histri in the North; the Etruscans, Latins, Falisci, Picentes, Umbri and Sabines in the Centre; and the Iapygian tribes (such as the Messapians), the Oscan tribes (such as the Samnites) and Greek colonies in the South.

The consolidation of Italy into a single entity occurred during the Roman expansion in the peninsula, when Rome formed a permanent association with most of other the local tribes and cities; and Italy's inhabitants included Roman citizens, communities with Latin Rights, and socii. The strength of the Italian confederacy was a crucial factor in the rise of Rome, starting with the Punic and Macedonian wars between the 3rd and 2nd century BC. As Roman provinces were being established throughout the Mediterranean, Italy maintained a special status with political, religious and financial privileges. In Italy, Roman magistrates exercised the imperium domi (police power), as an alternative to the imperium militiae (military power) exercised in the provinces.

The period between the end of the 2nd century BC and the 1st century BC was turbulent, beginning with the Servile Wars, continuing with the opposition of aristocratic élite to populist reformers and leading to a Social War in the middle of Italy. However, Roman citizenship was recognized to the rest of the Italians by the end of the conflict and then extended to Cisalpine Gaul when Julius Caesar became Roman dictator. In the context of the transition from Republic to Principate, Italy swore allegiance to Augustus and was then organized in eleven regions from the Alps to the Ionian Sea with more than two centuries of stability afterward. Several emperors made notable accomplishments in this period: Claudius incorporated Britain into the Roman Empire, Vespasian subjugated the Great Revolt of Judea and reformed the financial system, Trajan conquered Dacia and defeated Parthia, and Marcus Aurelius epitomized the ideal of the philosopher king.

With the development of provincial governments and the proliferation of citizenship, Italy gradually lost its position as the empire's heartland, though it retained the ideological value as Roman homeland. The Crisis of the Third Century hit Italy particularly hard, but the Roman Empire managed to survive and reconquer breakaway regions. In 286 AD, the Emperor Diocletian moved the imperial residence associated with the western territories (the later Western Roman Empire) from Rome to Mediolanum.^{[better source needed]} In 293 AD, Diocletian subdivided Italy into provinces and ended its special juridical privileges, which led to the loss of Italy's precedence over provinces. Meanwhile, the islands of Corsica, Sardinia, Sicily and Malta were added to Italy by Diocletian. The city of Rome declined as the center of power as new capitals were established outside Italy, such as Nicomedia, Sirmium, and later Constantinople. However, Italy remained the centre of the Western Roman Empire in late antiquity. Italian cities such as Mediolanum, Ravenna and Rome continued to serve as capitals for the West. The Bishop of Rome had gained importance gradually from the reign of Constantine the Great, and was given religious primacy with the Edict of Thessalonica under Theodosius I. Italy was invaded several times by the wandering Germanic peoples and fell under the control of Odoacer, when Romulus Augustus was deposed in 476 AD. Afterwards, Italy was ruled by the Ostrogoths and then briefly reconquered by the Byzantine Empire. The Lombard invasion in 568 AD would begin the fragmentation of Italy which lasted until its unification in 1861.

Following the end of the Social War in 87 BC, Rome had allowed its fellow Italian allies full rights in Roman society and granted Roman citizenship to all fellow Italic peoples. After having been for centuries the heart of the Roman Empire, from the 3rd century the government and the cultural center began to move eastward: first the Edict of Caracalla in 212 AD, extended Roman citizenship to all free men within the Imperial boundaries. Christianity then began to establish itself as the dominant religion from Constantine's reign (306–337), raising the power of Eastern metropolises, later grouped into Pentarchy.

Although not founded as a capital city in 330, Constantinople grew in importance. It finally gained the rank of eastern capital when given an praefectus urbi in 359 and the senators who were clari became senators of the lowest rank as clarissimi. As a result, Italy began to decline in favour of the provinces, which resulted in the division of the Empire into two administrative units in 395: the Western Roman Empire, with its capital at Mediolanum (now Milan), and the Eastern Roman Empire, with its capital at Constantinople (now Istanbul). In 402, the Imperial residence was moved to Ravenna from Milan, confirming the decline of the city of Rome (which was sacked in 410 for the first time in almost eight centuries).

The name Italia covered an area whose borders evolved over time. According to Strabo's Geographica, before the expansion of the Roman Republic, the name was used by Greeks to indicate the land between the strait of Messina and the line connecting the gulf of Salerno and gulf of Taranto (corresponding roughly to the current region of Calabria); later the term was extended by Romans to include the Italian Peninsula up to the Rubicon, a river located between Northern and Central Italy.

In 49 BC, with the Lex Roscia, Julius Caesar gave Roman citizenship to the people of the Cisalpine Gaul; while in 42 BC the hitherto existing province was abolished, thus extending Italy to the north up to the southern foot of the Alps. Under Augustus, the peoples of today's Aosta Valley and of the western and northern Alps were subjugated (so the western border of Roman Italy was moved to the Varus river), and the Italian eastern border was brought to the Arsia in Istria. Lastly, in the late 3rd century, Italy came to also include the islands of Sicily, Corsica and Sardinia, as well as Raetia and part of Pannonia. The city of Emona (modern Ljubljana, Slovenia) was the easternmost town of Italy.