The authors agree that this surname derives from the personal name Pietro, the Italian form of the Latin Petru(m), which in turn reproduces the Greek Pétros —and its feminine pétra—, with the meaning of 'rock, stone'. This Greek term translated the Aramaic Kepha, the original name of the apostle, also interpretable as 'rock'.

F. De Laurentiis notes that Pietro is the etymologically most faithful form, to which are added from the high Middle Ages the variant Piero and, later, the regional forms Petro, Pedro, Pierro, Perro, Pero and Pèiro. He adds that some surnames of this group may also reflect, at least in part, place names of the Pietra, Petra, Pedra or Pèra type.

E. Rossoni places Pietri as an infrequent surname, proper to the area between Reggio Emilia and Modena. He further notes that Pietro, in its underived form, is absolutely rare and seems to belong to southern Italy.