There are many hypotheses about the origin of this surname. The most known indicates that it is a reference of red hair. Among romans, this trait had originated the Latin cognomen Rossius, derived from the Latin rubius (red), and in some cases it can be concluded that the surname Rossi derives directly or through hypocoristic forms (or even dialectal) from this cognomen.

The authors agree that this surname derives from the nickname rosso ("red"), linked to the colour of the hair, beard or skin of the bearer. The predominant form in the north and centre of Italy is Rossi, while in the south it appears as Russo, and in Sardinia as Ruggiu or Ruiu. The ultimate etymon is the Late Latin adjective russus or rubius, a variant of the classical rubeus.

E. De Felice notes that in Sardinia the forms Ruggiu and Ruiu are more frequent in the Sassari area, and that in Friuli-Venezia Giulia there is the variant Russiani. He also documents apocopated forms proper to Veneto. E. De Stefani specifies that the use of the nickname rubeus or rubius goes back to Late Latinity and was maintained during the Late Middle Ages, the moment at which the surname became fixed. He further supplies historical documentation in the Carnia —a mountainous zone of northern Friuli-Venezia Giulia, in the province of Udine—, where the Rossi are recorded from the mid-14th century.

It is also considered that the nickname could allude to the colour of clothing, flags or insignia of political factions. F. De Laurentiis highlights that red had its own symbolic values in the Middle Ages, and that it even became a frequent personal name, which broadened the paths of formation of the surname.

Other complementary hypotheses suggest that: one links the surname to the Germanic Ross ("horse"), in reference to the reddish coat of the animal; another connects it with the Germanic name Rotz or Rutz, formed on the root hrod ("glory," "fame"), the same present in names such as Roberto, Rodolfo or Rolando. A possible toponymic or professional origin is also mentioned: in some areas, the Rossi would have been "those of the stream," for living near a brook —called roja in Slovenian or roggia in the Lombardy area—. In Trieste, for its part, the surname is related to the Slovenian form Rosic and the Croatian Rusic, whose suffix -ic means "son of."