The surname Rey has a presence above all in Piedmont (Turin, Salbertrand, Ronco Biellese) and in the Val d'Aosta (Courmayeur, Nus, Cogne), regions that formed part of the old Duchy of Savoy and where the Franco-Provençal language was spoken. In these alpine zones, the term rey (from the Franco-Provençal rè or rey, the equivalent of the Italian re) was used as a byname or nickname.

This byname referred to the "royal" or majestic behaviour of the ancestor: it was applied to people who stood out for their natural authority, their dignified bearing or their position of leadership within the community. In some cases it could allude to one who had held an important or symbolic office, acting as a "capo" or central figure in the local sphere.

The form Rey reflects the Savoyard and Francophone origin of the surname: the Savoy area and the Val d'Aosta were historically bilingual territories, where Italian, French and Franco-Provençal coexisted. For this reason the surname preserves the French spelling Rey, instead of the Italianized form Re.

In the south, especially in Naples, the surname may have a different origin, linked to the Spanish presence during the Aragonese and Bourbon period. However, its oldest and most genuine nucleus is found in Piedmont and in the Val d'Aosta, within the Savoyard and alpine cultural sphere.