The 34-year-old was a free agent having been released by the Toffees earlier this summer.
He has been training with the Black Cats in recent weeks and has now put pen-to-paper on a one-year deal.
Pienaar, who made almost 200 appearances for Everton over two spells, is reunited with David Moyes at the Stadium of Light.
Moyes brought the South African to England during his 11-year spell in charge at Goodison Park.