Dionysius of Halicarnassus (Ancient Greek: Διονύσιος Ἀλεξάνδρου Ἁλικαρνασσεύς, Dionýsios Alexándrou Halikarnasseús, ''Dionysios (son of Alexandros) of Halikarnassos''; c.  60 BC – after 7 BC) was a Greek historian and teacher of rhetoric, who flourished during the reign of Emperor Augustus. His literary style was atticistic – imitating Classical Attic Greek in its prime.

He is known for his work Rhōmaikē Archaiologia (Roman Antiquities), which describes the history of Rome from its beginnings until the outbreak of the First Punic War in 264 BC. Out of twenty books, only the first nine have survived. Dionysius's opinion of the necessity of a promotion of paideia within education, from true knowledge of classical sources, endured for centuries in a form integral to the identity of the Greek elite.

He was a Halicarnassian. At some time after the end of the civil wars he moved to Rome, and spent twenty-two years studying Latin and literature and preparing materials for his history. During this period, he gave lessons in rhetoric, and enjoyed the society of many distinguished men. The date of his death is unknown. In the 19th century, it was commonly supposed that he was the ancestor of Aelius Dionysius of Halicarnassus.

His major work, entitled Roman Antiquities (Ancient Greek: Ῥωμαϊκὴ Ἀρχαιολογία, Rhōmaikē Archaiologia), frequently abbreviated Ant. Rom. (Latin: Antiquitates Romanae), narrates the history of Rome from the mythical period to the beginning of the First Punic War in twenty books, of which the first nine remain extant while the remaining books only exist as fragments, in the excerpts of the Roman emperor Constantine VII Porphyrogenitus and an epitome discovered by Angelo Mai in a Milan manuscript. Dionysius is the first major historian of early Roman history whose work is now extant. Several other ancient historians who wrote of this period, almost certainly used Dionysius as a source for their material. The works of Appian, Plutarch and Livy all describe similar people and events of Early Rome as Dionysius.^{[citation needed]}

In the preamble to Book I, Dionysius states that the Greek people lack basic information on Roman history, a deficiency he hopes to fix with the present work.

Because his prime objective was to reconcile the Greeks to Roman rule, Dionysius focused on the good qualities of their conquerors, and also argued that – based on sources ancient in his own time – the Romans were genuine descendants of the older Greeks. According to him, history is philosophy teaching by examples, and this idea he has carried out from the point of view of a Greek rhetorician. But he carefully consulted the best authorities, and his work and that of Livy are the only connected and detailed extant accounts of early Roman history.

Dionysius was also the author of several rhetorical treatises, in which he shows that he had thoroughly studied the best Attic models:

The last two treatises are supplemented by letters to Gn. Pompeius and Ammaeus (two, one of which is about Thucydides).