The surname Fossati has a toponymic origin: it derives from the Latin fossa, "ditch, excavation", or from Italian place names such as Fossato, Fossano, Fossale, etc. It is attributed to people who lived near a depression of the terrain, a canal or a ditch. In some cases, it originates from localities with similar names, used both as names of a city and of small rural sites. According to R. Brondolo it alluded to "inhabitants near a hollow or dell".

Variants of the surname identified in Italy: Fossa, Fosso, Fossi, Fossali, Fossani, Fossarèlli / Fossarèllo, Fossato, Fossano. These surnames can be interpreted as "those who lived beside a ditch" or "coming from a place called Fossato".

As for the geographical distribution, Fossati has more than 3,300 families, of which 60% reside in Lombardy, especially in Lissone (Brianza). It is also very present in Piedmont and Liguria. It is the most frequent surname in Mozzanica (Bergamo) and ranks 15th in the province of Alessandria. A lineage is also recorded in Florence, probably linked to the place name Fossato. There are minor nuclei in the Veneto: Campagna Lupia (Venice), Vicenza and Monteforte d'Alpone (Verona). In Novi Ligure, Fossati reaches 32% of the local population, and it also appears in Gavi and Berzano di Tortona.

As for the variants: Fossa is typical of Liguria; Fossi predominates in Tuscany; Fossani is found especially in Piedmont; Fosso has presence in Campania.

Regarding historical documentation, the first documented Fossati is a knight of Lissone (Brianza) who appears between 1229 and 1230 in the troops of the Comune of Alba (Piedmont): Scottus de Fossato de burgo Lixono. In the year 1237, Fossati families are recorded in Lissone, Cologno Monzese, Missagliola, Cremella, Vedano and Monza. In the Codice Diplomatico della Lombardia medievale phrases appear such as a mane fossatus, Gandulfinus … a sera fossatus. Martinus penus …