The authors agree that the surname derives, through dialectal modifications —including hypocoristic forms—, from the medieval name Thomas or Thomasus. This name comes from the Hebrew root ta'am, meaning "twin". Among its best-known historical bearers are the apostle Thomas —whom the evangelist John calls Didymos, which means precisely "twin"— and Thomas Aquinas (1225–1274), Doctor Universalis of the Catholic Church.

E. Rossoni documents the surname in a diploma of King Charles I of Anjou signed in Capua in August 1267, as well as in deeds of 1373, 1447 and 1476. He further notes that the variants Tomasi and Tommasi are diffused throughout northern Italy, with a presence also in the Cagliari area, the Salento area —that is, the southernmost zone of the Apulia region, in southern Italy— and in southern Sicily. The variant Tomassetti predominates in the central strip of the Italian peninsula, especially in Marche, Umbria, Lazio and Abruzzo. Tomassi is concentrated in Umbria, Lazio and the L'Aquila area, with the greatest density in Rome, the Frosinone area and Palestrina. Tomasso, for its part, appears in the Frosinone area, Rome and Molise.

F. Miranda offers a broader view of the geographical distribution: the surname is diffused throughout Italy. It predominates in the north —Trentino-Alto Adige, Veneto, Lombardy, Emilia-Romagna, Piedmont and Friuli-Venezia Giulia—, but is also frequent in the centre and south: Sicily, Sardinia, Lazio, Liguria, Tuscany, Campania, Apulia and Abruzzo. In Sicily it is present in almost all the provinces, with a special concentration in the Syracuse, Ragusa, Catania, Caltanissetta and Messina areas.