The authors agree that this surname is of toponymic origin, derived from the numerous place names containing the root piazza or piazzo, widely diffused throughout Italy. Among the places that gave rise to them are Piazza Armerina, Piazza Brembana, Piazzola sul Brenta, Piazza al Serchio and Piazzatorre. F. De Laurentiis and E. De Felice further note derived forms such as Piazzola, Piazzolo, Piazzolla and the demonym Piazzese.

E. De Stefani adds that the term refers to those who lived near the main square of the village, a designation that in time became a surname. He also documents variants arisen in German-speaking communities of Friuli: Plozner, Plozzer, Plocer, Plocir, Platzer and Platzner, which reflect the local word for designating the square. A document of 1717 from Sauris di Sopra mentions a locco detto hauf der Plocce ('over the square'). These forms appear in localities such as Timau, Paularo and Sauris, and their equivalents Platzer and Platzner are also documented in the south of Austria.

E. Rossoni specifies the distribution by variant: Piazza is very common throughout the north, in the Naples area and in Sicily; Piazzi is typical of the area comprising the Milan, Bergamo and Cremona areas in Lombardy, Trentino-Alto Adige, the Verona area, and in Emilia the Bologna and Ferrara areas; Piazzo, very rare, shows an area of presence in Piedmont —the Turin, Cuneo and Asti areas— and another in Veneto —the Vicenza and Padua areas.

F. Miranda notes that in Sicily the surname has a greater presence in the areas of Agrigento, Palermo, Trapani, Catania, Messina, Caltanissetta and Enna. G. P. Holmes proposes an alternative etymology from medieval Latin with the sense of 'market', linking it to the city of Piazza Armerina.