Corndale's new Redback
Cooper, who has been on a NSW Rookie contract for several seasons, was not going to be offered a contract with the Blues this season following a lean trot with the bat in Sydney first grade.
However, South Australia approached the talented right-hander with an offer he couldn't refuse.
"Rodney Marsh invited him down for a trial a while ago and he spent a day down there," Tom's father, Barry Cooper, said.
"He didn't get much feedback but he eventually got an email that said South Australia would offer him something and to be patient.
"They were all finalising budgets and players, and then they offered Tom a full contract.
"It's a lucky break and he couldn't say no to a full contract."
It's believed SA selectors took notice of Cooper when he hit 112 for the NSW Second XI against the South Australian Second XI in Adelaide last October.
Although a Second XI match, the SA team included eight players with Sheffield Shield experience including paceman Mark Cleary and spinner Cullen Bailey.
The contract means the 21-year-old can concentrate fully on his cricket while not being distracted by work.
It's a performance-based agreement that will pay more if he plays top-level cricket in either the Sheffield Shield, Ford Ranger Cup (one-day) or Twenty20 competition.
The former Australian Under-19, Under-17 and NSW Country representative will be joined by Michael Klinger and Aaron O'Brien as the new interstate players signed by South Australia.
He will join former University of NSW team-mate Dan Christian in the squad.
Cooper will link up with his Redbacks team-mates in Adelaide in September because he is now playing in Scotland with Forforshire, near Dundee.
"He just wanted to get out of Sydney for a while and it's worked out well," Barry Cooper said.
"He's doing a development officer's job there (in Scotland), although he's a bit concerned as to whether the sun exists in that part of the world."
Apparently, half of last season's games were washed out.