In addition, MasterChef Junior has also been renewed for another season.
"MasterChef has been one of my favorite shows since it began, and like so many others, I fell in love with MasterChef Junior," Tosi, a renowned pastry chef and owner of New York City's Milk Bar, said in a statement.
"I'm incredibly humbled by the opportunity and excited to pay forward all that I owe to learning what you're made of in the home kitchen, and putting your neck on the line to show what you've got to the rest of the world!"
"Christina is one of the most exciting, dynamic and inspirational people in food today, whose brilliant ideas and incredible work ethic have resulted in a meteoric rise in the culinary world," added Ramsay. "I have tremendous respect for her opinions, and she'll be an incredible judge and phenomenal mentor to the best home cooks in America."
MasterChef Junior's third season premiered on Fox earlier this month. Production on Season 4 will begin this spring.
The sixth season of the original MasterChefseries --the first to feature Tosi -- is currently in production and will air this summer.
MasterChef and MasterChef Junior are produced by Endemol Shine North America and One Potato Two Potato. Elisabeth Murdoch, Eden Gaha, Paul Franklin, Robin Ashbrook, Gordon Ramsay, Adeline Ramage Rooney, Patricia Llewellyn, and Ben Adler serve as executive producers.