But it was one of the closest wins he could possibly get.
Election returns in Finland indicate the Social Democratic Party was the top vote-getter in the country's parliamentary election, trailed closely by the populist euroskeptic Finns Party. Mr Rinne said it would be hard to work with the Finns Party. The National Coalition, in turn, came to within 0.5 percentage points of the Finns Party, becoming the third largest party with 38 seats in the Parliament.
The ruling Center Party secured 13.8% of the vote and the opposition Green League party - 11.5%.
The party could hold significant influence in the talks to form the next government, which in Finland is typically a coalition of three or four parties.
Finland is slated to take over the rotating European Union presidency from Romania in July.
As required by Finnish law, election officials asked the first voter to confirm the ballot box was empty before it was locked and voting could begin.
And Petteri Orpo, leader of the conservative National Coalition Party and co-architect of the government's savings programme, has denounced the Social Democratic Party's anti-austerity plans as "irresponsible".
Yet after four years of spending cuts under the current administration, there is little appetite among the public for further belt-tightening.
Political analyst Sini Korpinen told AFP that, in a bid to keep the nationalists in opposition, a coalition between the leftist Social Democrats and the centre-right National Coalition appeared nearly a certainty. "We need to have a government where there is the same value base".
Finland is the first country to complete a nation-wide experiment on universal basic income.
He told reporters on Sunday, "I could not expect a result like this, and no one could". After the elections, the composition of the 200-seat unicameral parliament will be determined, a new government will be formed and a prime minister will be appointed.
That means Finland went to the polls under a caretaker government, a move which was derided as a political stunt by Sipila's opponents.
The Finns Party more than doubled its seats to 39.
The next Finnish government is predicted to be formed on the basis of Social Democrats' cooperation with either the conservatives or the centrists.
Finland has a rapidly ageing population and declining birth rate, and the question of how to keep funding the country's generous welfare state has been a key election battleground.
Immigration became a hot election topic following outrage in January over highly publicised reports of an alleged string of sexual assaults by immigrant men.
"The Finns Party has had an effect on the way that we speak about immigration", political commentator Sini Korpinen said.
Meanwhile, the Finns Party, which won 39 seats, had focused nearly entirely on an anti-immigration agenda under the leadership of hardline MEP Jussi Halla-aho, who also decried the "climate hysteria" of the other parties.