Steven Marshall has been sworn in as South Australia's 46th Premier, alongside the state's first female Deputy Premier Vickie Chapman and new Treasurer Rob Lucas.
The conservative Liberal Party has won the election of South Australia state, ending the Australian Labor Party's streak of 16 years in power in the state.
Mr Weatherill visited Government House late on Sunday morning to officially resign as premier, where he told reporters he planned to see out his term in the South Australia State Parliament.
Speaking briefly out of an open auto window as he left, Marshall told reporters the premiership was "a great honour and a great responsibility, but a great opportunity - a great opportunity, I'm really looking forward to it".
"I give you my commitment that I and every single person of the team will be working diligently, every day, over the next four years so we can build a brighter future for South Australia", he said.
"This is an incredible opportunity".
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Mr Marshall said job creation would be a focus of his government and also pledged to lower taxes and the cost of living.
Opposition Leader Steven Marshall's Liberal Party was projected to win 24 seats in the 47-seat parliament by ABC election analyst Antony Green.
Under premier Jay Weatherill, the state of South Australia had become an worldwide laughing stock.
Prime Minister Malcolm Turnbull offered his congratulations to Mr Marshall.
The established parties-the centre-right Liberals and Labor-face a huge challenge from a new minor party, SA Best, led by ex-federal independent senator, centrist Nick Xenophon.