The 31-year-old beauty gave birth to her son, Noah, on Monday at the Santa Helena Clinic in Florianopolis, Brazil.
The proud papa, a California businessman, confirmed the birth of his son via Twitter, saying, "Welcome my son, can't wait to show you around this crazy place."
Ambrosio's mother-in-law, Michelle Mazur, also wrote on her Facebook page Monday, "Grandson just born in Brazil!"
The baby's birth comes just two days after the Victoria's Secret model's baby shower.
Alessandra and Jamie are already parents to 3-year-old daughter, Anja.
Brazilian supermodel Alessandra Ambrosio and longtime fiancé Jamie Mazur have welcomed their second child, a baby boy!