E. De Felice documents this surname as derived from a nickname or occupational name, Martèllo, recorded as early as Farfa (Sabina) in the year 1011 under the form Johannes Martellus. The term goes back to one who uses the hammer by trade or in war, or to one who strikes with great force, an evocation reinforced by the prestige of Charles Martel and, in the 13th century, of Charles Martel of Anjou. De Felice further notes that in some cases it may reflect the hypocoristic of Martino, and that the feminine forms in -a could derive from Marta.

As for the geographical distribution, De Felice places Martelli with a high frequency in Emilia-Romagna and Tuscany, while the forms Martello and the variants in -a predominate in southern Italy, Martiello is proper to Naples, and Martellani and Martellato are concentrated in Veneto.

G. P. Holmes proposes an alternative origin and links the surname both to the given name "Marte" or its derivative Martino and to the sense of "he who uses the hammer". P. Canè, for his part, points in a similar semantic direction and suggests that the surname could have alluded to a tenacious character or else to the trade of blacksmith.