The Socialists, however, are 10 seats shy of a majority in the 230-seat parliament. To ensure that its legislation is approved, the government will need support from other parties.
A healthy economy, with growth of 2.1 % last year and unemployment at around 6%, helped the Socialist Party back into the Sao Bento Palace, the seat of power in Lisbon.
The Socialists have also earned praise around Europe for taming Portugal's chronic overspending, with Finance Minister Mario Centeno bringing the budget deficit close to zero this year.
Antonio Costa, the Socialist leader and incumbent prime minister, said in his victory speech that he intends to continue his business-friendly policies and prudent fiscal management.
But economic troubles are brewing in Europe, and once the election results were known the radical Left Bloc and Portuguese Communist Party — the most obvious candidates for an alliance — were quick to issue demands for more spending.
The Left Bloc snared 19 seats and the Communist Party got 12.
Left Bloc leader Catarina Martins said her party's demands in return for support would include the re-nationalization of the national postal service CTT, which was privatized in 2014, and a boost in public investment in housing and transport.
The Communists want the minimum monthly wage to rise to 850 euros ($932) from 600 euros, an increase in pensions and free pre-school for all children up to three years old, among other things.
Antonio Barroso, deputy director of research at London-based consultancy Teneo, said he didn't expect many changes in the country's political path.
"Regardless of the final configuration of the government, it is unlikely that the Costa II administration will implement significant economic reforms," he said.
The next government faces plenty of economic challenges.
Apart from signs of a broad slowdown in the European Union, looming on the horizon is the United Kingdom's impending departure from the EU, now scheduled for Oct. 31. The U.K. is one of Portugal's main export markets.
Portugal also has a government debt equivalent to more than 120% of its gross domestic product — the third highest in the EU — so any interest rate rise could be deeply damaging.
The country's aging population, meanwhile, is threatening the financing of Portugal's welfare system. The EU says at current fertility rates, the number of Portuguese will decline from 10.3 million this year to 6.6 million in 2100.
Climate change is another problem, especially in the southern half of the country where rainfall is increasingly scarce.
Three relatively new parties also earned seats in parliament, including right-wing populist party Chega, which got more than 66,000 votes from the 10.8 million eligible voters and a seat in the Republican Assembly, Portugal's parliament.
It may take several weeks of negotiations before a new government takes office.