The younger brother of the UFC lightweight champion will face Ryan Benoit at UFC 185.
UFC officials announced the news on Tuesday.
Sergio once fought as a flyweight outside the UFC, so the move is not entirely unexpected.
With the addition, UFC 185 now includes:
Anthony Pettis vs. Rafael dos Anjos
Johny Hendricks vs. Matt Brown
Roy Nelson vs. Alistair Overeem
Chris Cariaso vs. Henry Cejudo
Ryan Benoit vs. Sergio Pettis