The 29-year-old will move to Franklin's Gardens from Perth-based Super Rugby franchise Western Force.
He becomes Saints' second major front-row signing ahead of next season after they swooped last month for London Irish and England prop Alex Corbisiero.
Ma'afu has won 14 caps and was part of Australia's 2011 World Cup squad, but a broken arm suffered during last year's Super Rugby campaign meant he missed the inaugural Rugby Championship and Australia's autumn tour to Europe.
"We have been impressed by what we have seen from Salesi," said Northampton rugby director Jim Mallinder.
"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.
"Salesi will add a great deal to our group of props and the squad in general going into next season, and we are looking forward to welcoming him to Franklin's Gardens later in the year."