The boy, born at the Losenets hospital in Sofia, is the first child of Kalina, 35, and Spanish explorer Kitin Munoz, whom she married in 2002.
He is the 11th grandchild of former King Simeon Saxcoburggotski, who was exiled in 1946 at the age of 9 by the communist regime ruling at the time.
After spending decades in Spain, Saxcoburggotski returned to Bulgaria in 2001 and became prime minister after his centrist party won in a landslide election victory.
In 2005, his party lost elections to the Socialists, but remained part of a coalition government led by the Socialists, reports AP.
Kalina is Saxcoburggotski's only daughter with Spanish wife, Margarita. The couple has four sons, and lives in Sofia.