The authors agree that Costamagna is a surname of toponymic origin, derived from the homonymous locality that is a frazione of the municipality of Lequio Tanaro, in the Cuneo area. The name combines two Latin elements: costa (from the Latin costa) and magna (from the Latin magnus, "great"), with the approximate sense of "greater terrain" or estate of great extension. It is a Piedmontese archaism that reflected the medieval practice of assigning bynames according to the geographical origin or the characteristics of the family territory.

The surname became such from the name of that hamlet, following a custom peculiar to the Late Middle Ages in Italy. The region where it is integrated most fully is Piedmont, with especially strong concentrations in the Cuneo area. Significant presence is also recorded in Liguria, Emilia-Romagna and Lombardy, the result of internal migrations.

E. Giraudo agrees on the origin and the etymology, and highlights that from that small locality arose one of the most representative surnames of the Cuneo area, with greater presence in municipalities such as Fossano, Cuneo, Cherasco and Lequio Tanaro itself.