E. Rossoni notes that this surname derives from the medieval Italian name Guelfus. The variant Guelfi, fairly rare, is distributed along the coast from Genoa towards the La Spezia, Massa, Pisa and Livorno areas. The variant Guelfo, extremely rare, has one presence in the Genoa area and another in that of Turin.

V. Blunda adds that the surname is of Germanic root, derived from the personal name Welf, linked to the Welf lineage, a Germanic dynasty originally from Swabia which gave its name to the Guelph faction of communal medieval Italy. The clash between Guelphs —supporters of the pope— and Ghibellines —supporters of the emperor— structured the political life of the Italian republics during the 13th and 14th centuries, and appears as a historical backdrop in Dante Alighieri's Divine Comedy, Dante himself belonging to the Florentine Guelph faction before his exile in 1302. Blunda records it as a surname proper to Sicily.