The 30-year-old right-hander, who has played 58 Tests for Pakistan, is expected to arrive in time for the start of the Australian domestic season in October.
"Securing a batsman of Younis' calibre will provide our young South Australian batsmen with unprecedented experience, and our top order are really excited by the news," South Australian coach Mark Sorell said.
"Younis is currently one of the world's best batsmen, recruiting him is an important step in our recruiting strategy and an opportunity that presents itself as too good to miss.
"We expect Younis will play a key role in making sure the Redbacks (South Australia) are competitive this season and ensuring our bowlers have good targets to work with."
Under the terms of his contract Younis will play with South Australia until Christmas and then possibly return to South Australia for the finals of the domestic competitions, if required, SACA officials said.
Officials said his commitments with South Australia will not impinge on his playing commitments with Pakistan.
Younis has scored 4,816 Test runs at 49.14 with 15 centuries since his Test debut in 2000.