The 29-year-old, whose career has been blighted by injuries, will join the Saints at the beginning of next season.
"We have been impressed by what we have seen from Salesi. He has built up a good reputation as one of the top props in Australia and has been a big part of the Wallabies' recent improvements in the set piece," Mallinder told the club's official website.
"Salesi will add a great deal to our group of props and the squad in general going into next season, and we're looking forward to welcoming him to Franklin's Gardens later in the year."
Ma'afu has also played for Brumbies.