Dennis or Denis is a first or last name from the Greco-Roman name Dionysius, via one of the Christian saints named Dionysius.

The name came from Dionysus, the Greek god of ecstatic states, particularly those produced by wine, which is sometimes said to be derived from the Greek Dios (Διός, "of Zeus") and Nysos or Nysa (Νῦσα), where the young god was raised. Dionysus (or Dionysos; also known as Bacchus in Roman mythology and associated with the Italic Liber), the Thracian god of wine, represents not only the intoxicating power of wine, but also its social and beneficent influences. He is viewed as the promoter of civilization, a lawgiver, and a lover of peace—as well as the patron deity of both agriculture and the theatre.

Dionysus is a god of mystery religious rites, such as those practised in honour of Demeter and Persephone at Eleusis near Athens. In the Thracian mysteries, he wears the "bassaris" or fox-skin, symbolizing new life. (See also Maenads.)

A mediaeval Latinised form of the Anglo-Norman surname Le Denys was Dacus, which correctly meant Dacian, but when the Vikings were about was often used to mean "Danish" or "The Dane". The name became modernised as Denys, then later as Dennis.

Alternative forms and spellings of the name include Denis, Denys, Dennys, Denish, Deon, Deonne, Deonte, and Dion, Dionice. Diminutive forms include Den, Dennoh, Deno, Denny, Deny and Deen.

The name Sydenie (alternate spellings: Sydney or Sidney) may derive from a village in Normandy called Saint-Denis. A medieval diminutive was Dye, from which the names Dyson and Tyson are derived.

Dennis is a very popular English, Irish and Danish name, common throughout the English-speaking world, and a very popular French name, common throughout the Francophone world. It is also common as a German, Italian, Dutch, Croatian, Belarusian, Ukrainian, Russian, Bulgarian, Brazilian, Bosnian, and Albanian name.

Dionizy is the Polish version of the name, while Dionigi and Dionisio are the Italian versions of the name. The Irish name Donnchadh is sometimes anglicised as Denis, but has a different origin and is in fact related to the names Duncan and Donagh. Feminine versions of the name include: Denise, Denisa, Deni, Denice, Deniece, Dione, and Dionne.