E. De Felice, E. Rossoni and F. De Laurentiis agree that the surname was formed from nicknames and trade names derived from sardella or sardina ("sardine"), arisen from the High Middle Ages to denominate thin and lean people, or fishermen and sellers of sardines and other fish.

De Felice specifies that the term also includes the regional forms sardéna, of the north, and sardóne, of the lower and middle Adriatic. He notes that Sardelli may also belong to the Sardi surname group.

Rossoni suggests instead as the main origin the medieval name Sardellus, documented in the 14th century in a writing of Benvenuto da Imola, although he also admits the derivation from the trade of fisherman or from physical characteristics of the founder of the line. He documents the transformation into a surname in Florence in the 16th century, in the autobiography of Benvenuto Cellini.

Rossoni further distinguishes the forms: Sardella is of Apulian origin, with possible secondary nuclei along the Adriatic coasts; Sardelli has a Tuscan nucleus in the area of the Florence, Siena, Pisa and Livorno areas, another between the Rome and Frosinone areas, and another between those of Brindisi and Taranto; Sardello is practically unique.

De Laurentiis adds that the surname is diffused from the south to the centre.