Mr Weatherill was unanimously endorsed as new leader at a meeting of the Labor caucus in Adelaide today, following the resignation of Mike Rann.
He'll officially become the state's 45th premier when he's sworn in by Governor Kevin Scarce later this morning.
Chloe Fox and Ian Hunter have been elected to fill the ministerial vacancies left by Mr Rann and former treasurer Kevin Foley, who's also stepped down from cabinet.
Transport Minister Patrick Conlon said Mr Weatherill's appointment came amid great enthusiasm from Labor MPs.
"Jay Weatherill is the new leader of the Labor Party and the first new leader in 17 years," he told reporters at parliament house.
"That was a very warmly welcomed decision."
Mr Rann did not attend Friday's caucus meeting.
Mr Weatherill will also reveal his new cabinet on Friday, with several ministers expected to be given new or revised responsibilities.
He is scheduled to hold a media conference on Friday afternoon.
Ahead of the caucus vote Labor backbencher Steph Key said she believed both Ms Fox and Mr Hunter would make excellent ministers.
She said she also expected new enthusiasm from the government with Mr Weatherill as premier.
At 47, Mr Weatherill is more than 10 years younger than Mr Rann but has been a minister since Labor was returned to office in 2002.
He is seen as a more conciliatory figure and has already indicated he will bring a more consultative approach to government.
