Pienaar, who was released by Everton at the end of last season, has completed a successful fitness trial and earned himself a one-year deal with the the Black Cats.
The 34-year-old Westubury-born midfielder will reunite with former Toffees boss David Moyes at the Stadium of Light for the current seaon.
The former Bafana Bafana captain could make his Sunderland debut in their Premier League clash at home to Middlesbrough on Sunday.
The move dispels reports of a possible return to South Africa as both Orlando Pirates and Mamelodi Sundowns attempted to land the former Ajax Amsterdam playmaker for the 2016/17 season.