It will be Pacquiao's first fight since he was knocked out by Juan Manuel Marquez in December, a defeat that prompted many to suggest the 34-year-old should retire after being left unconscious, face down on the canvas by his Mexican rival.
However, Pacquiao, winner of world titles at eight different weight divisions, has ignored the calls to quit and after the lengthy break will fight at the Cotai Arena in The Venetian Macao-Resort and Hotel, Fred Sternburg said.
It will be a first bout outside the United States since 2006 for Pacquiao 54-5-2 (38 knockouts), who also lost to Timothy Bradley in his only other bout last year.
The back-to-back defeats have seemingly ended all hope of a lucrative clash against undefeated American Floyd Mayweather Jr.
Rios 31-1-1 (23 knockouts) is also coming off a loss, Mike Alvarado claiming a unanimous points decision in their March contest in Las Vegas.
Pacquiao's trainer Freddy Roach predicted a swift end to the non-title bout.
"Manny is definitely going to get his mojo back with this fight," Roach told ESPN.com.
"I think it'll be an exciting fight but that Manny is going to land ... and knock this guy out somewhere along the way. Rios likes to exchange and his style is not really difficult to figure out, so Manny's gonna hit him." (Reporting by Patrick Johnston in Singapore; Editing by Peter Rutherford)