The promotion confirmed the booking on Thursday.
The two were originally set to scrap at this weekend's UFC 149 until an undisclosed injury to Aldo forced him to withdraw from the date.
The champion has won 14 straight fights dating back to 2006. He first won the belt when it was still a WEC title, back in 2009, and has successfully defended it five times, most recently knocking out Chad Mendes at January's UFC 142.
His next defense will also come in front of the same Rio fans, as UFC 153 is set for the same host venue, the HSBC Arena.
Koch (13-1) comes into the bout on the strength of a four-fight win streak, with consecutive victories over Jonathan Brookins, Raphael Assuncao, Francisco Rivera and Bendy Casimir. Three of those wins have come by finish.
The event, which takes place on October 13, will mark the promotion's third visit to Rio in just over one year, and fourth overall to Brazil in that time.