What Exactly Are San Marzano Tomatoes? These tomatoes are grown under very strict rules and have the official DOP stamp guaranteeing authenticity. Each can has an assigned number. But most importantly, San Marzano also refers to that specific variety of tomato grown in a specific region of Italy: the Agro Sarnese Nocerino of the Sarno River valley near Mount Vesuvius, where volcanic soils are said to produce sweeter, less acidic tomatoes. These are “True Authentic” San Marzanos, and their unique combination of varietal and terroir is so special that they’re given official status as a “protected designation of origin” or D.O.P. (Denominazione d’Origine Protteta). When it comes to canned tomatoes, one name reigns supreme, Nicastro San Marzano DOP.