A humbled Premier Peter Malinauskas said that although it was the best result his party has ever achieved, it's was important that no one confused the result as adulation.The victory should be an seen as an invitation to continue to "work out guts out for the next four years", the premier said.
Of the state's 47 seats, Labor had secured 30, the Liberals had 4, with 13 seats still in doubt.
Late on Saturday night, Electoral Commission figures showed statewide Labor had 37.8 per cent of the vote, One Nation had 21.7 per cent, the Liberals slipped to third on 19.1 per cent and the Greens were on 11.6 per cent.
Liberal leader Ashton Hurn, who stepped into the role 103 days before the election, said it was a tough night for her party.
"Now is the time to celebrate the Liberals, the future, because there's hope for the Liberal future here in South Australia," she told supporters.
"The voters didn't give us the nod this time, we'll keep fighting into the future. The voters never get it wrong, and they've sent us a clear message."
One Nation leader Pauline Hanson said it was the start of a new beginning for the people of South Australia and people of Australia.
The party was hoping to win three seats, she said, including state leader Cory Bernardi's in the upper house.
But it remained unclear if the party would win any lower house seats, with complex preference deals set to decide the result in several country seats.
ABC election analyst Casey Briggs said the result was "the disintegration of the two party system".
"It's a political earthquake we're watching in South Australia tonight," he said.
"SA Labor has had its best result in the House of Assembly on record and at the same time we have a massive disruption on the right of politics."
Ms Hanson described One Nation elected members as "landmines".
"I suggest don't step on them because that's what we're here for, to ensure you get good representation, good government with a good opposition which you have not had for a long time."
Mr Bernardi said he was because "an earthquake has rattled the foundations of uni-party politics in South Australia".
"I promise you there is more to come," he said.
"One Nation will have the strongest voice in the South Australian parliament that it has ever had."
In some seats, the Liberals slipped to fourth behind Labor, One Nation and Greens candidates.
Former Liberal leader Vincent Tarzia lost his seat of Hartley to Labor's Jenn Roberts.
Mr Tarzia's predecessor, David Speirs, who was convicted of drugs charges last year, failed in his bid as an independent to regain his seat of Black from Labor's Alex Dighton.
Retiring Mawson MP and Speaker Leon Bignell said it was a big win for Labor, with great candidates taking seats it hadn't held for a fair while.
"I don't think we'll see any hubris from Peter. He's a hard worker who doesn't like to see people taking anything or anyone for granted," he said.
On the rise of One Nation, he said there was a lot of people certainly disillusioned, particularly with the Liberal Party.
"It's a reality that the Labor Party and the Liberal Party are going to have to deal with," he said.
"There's no point ignoring any of these people, their votes equal to everyone else's vote, and they've got a point of view, and we always do better when we listen to everyone's point of view."
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