The welterweight will once again have the support of a home crowd, with the bout confirmed for the Motorpoint Arena Sheffield.
Brook said that he intends to improve on his last showing, a hard-fought points victory over tough American Carson Jones.
The 26-year-old said: "I proved I have massive heart against Carson but I said after that fight there needs to be an improvement.
"On October 20 my fans are going to see me at my very best."
A win for Brook against Saldivia secures a fight for the IBF crown against the winner of a clash between Randall Bailey and Devon Alexander on Saturday, September 22.