The sources consulted agree that this surname derives from the given name Napoleone, whose origin is debated. The most widespread hypothesis links it to Tuscany and interprets it as a dialect form produced by syncope of the compound name Iacopo Leone.

A. Bongioanni develops this etymology in detail: in the element Napo — frequently used as a shortened form — he sees a Tuscan variant of Lapo, so that the name would be the equivalent of Iacopo Leone. In support of his thesis he cites the surnames Pierleoni, Gianleoni, and Ponzileoni, and notes that the Tuscan forms Nappi and Nappini reinforce his interpretation. Bongioanni expressly rejects the etymology that derives the name from Naples, proposed by Rajna in 1891, despite its being widely circulated. He further indicates that the name spread from Tuscany toward northern Italy, and records a Venetian dialect form, Poleon, and a Piedmontese one, Napione.

E. Rossoni records an alternative hypothesis, according to which Napoleone could derive from an Italian adaptation of the Germanic term nibelung, referring to the mythological dwarf from whom the Burgundian kings are said to have descended, though he considers this etymology less accepted. This same Germanic origin is the one proposed by the source Surnames in Italy, which connects the name to the Germanic given name Nibelung, derived from nebel, meaning "fog."

Regarding geographical distribution, Rossoni notes that the form Napoleone appears in Cagliari, in Rome and its surrounding area, in Abruzzo — especially in the areas of Pescara and Chieti — and in the areas of Naples and Bari. The variant Napoleoni is typical of Lazio, with a strong presence in Rome, and extends to the areas of Perugia, Macerata, and Ascoli Piceno.